What Happens If You Miss the CT600 Deadline? Penalties, Interest & How to Fix It

What Happens If You Miss the CT600 Deadline? Penalties, Interest & How to Fix It

Missing your CT600 deadline is one of the most stressful things that can happen to a company director. Whether you forgot, didn't realise you had a deadline, or simply ran out of time — the consequences are real, and they escalate fast.

The good news? You can fix it. And the sooner you act, the less it'll cost you.

In this guide, we'll walk through exactly what happens when you miss your CT600 filing deadline — day by day, month by month — including the specific penalty amounts, how interest is calculated, and the concrete steps you need to take right now to sort it out.

First Things First: When Is Your CT600 Deadline?

Your CT600 (Company Tax Return) must be filed with HMRC within 12 months of the end of your company's accounting period. This is separate from the corporation tax payment deadline, which is 9 months and 1 day after your accounting period ends.

For example, if your company's financial year ends on 31 March 2025:

• Corporation tax payment deadline: 1 January 2026
• CT600 filing deadline: 31 March 2026

These are two different deadlines with two different sets of penalties. Miss either one and HMRC will come knocking.

Not sure when your deadline is? Read our complete guide to CT600 deadlines to work out your exact dates.

The CT600 Late Filing Penalty Structure (Exact Amounts)

HMRC doesn't mess about with late filing penalties. They're automatic — no warnings, no grace period, no "we'll let you off this time." The moment you're one day late, the first penalty hits.

Two penalty regimes apply: From 1 April 2026, HMRC doubled the fixed CT600 late filing penalties. The amounts that apply to your return depend on your CT600 filing deadline — not the date you actually file. If your accounting period ended on or after 31 March 2025, your filing deadline falls on or after 31 March 2026, and the higher amounts apply.

Here's the full breakdown:

1 Day Late: £100 or £200 Fixed Penalty

The second your CT600 is overdue — even by a single day — HMRC issues an automatic fixed penalty. There's no human review. It's computer-generated.

• Filing deadline before 1 April 2026: £100
• Filing deadline on or after 1 April 2026: £200

This penalty applies whether you owe £0 or £100,000 in corporation tax. Even companies with no tax to pay get hit with this.

3 Months Late: Another Fixed Penalty

If your CT600 still hasn't been filed after 3 months, HMRC adds a second penalty of the same amount:

• Filing deadline before 1 April 2026: +£100 (£200 total in fixed penalties)
• Filing deadline on or after 1 April 2026: +£200 (£400 total in fixed penalties)

Again, this is automatic. HMRC won't send you a warning letter between the first and second penalty — though you will receive penalty notices in the post and through your Government Gateway account.

6 Months Late: 10% of Unpaid Tax

This is where things get serious. At the 6-month mark, the penalty jumps from fixed amounts to a percentage of your unpaid corporation tax.

HMRC charges 10% of the estimated tax unpaid at that date. If you owe £5,000 in corporation tax, that's an additional £500 penalty — on top of the fixed penalties you've already accumulated.

Important: HMRC will estimate your tax if you haven't filed. They often estimate high. You could end up with a penalty based on an inflated figure until you actually submit your return and replace their estimate with your real figures.

12 Months Late: A Further 10% of Unpaid Tax

If a full year passes without filing, HMRC charges a further 10% of the estimated unpaid tax — bringing the total tax-geared penalty to 20% across the two stages combined. This is not a single 20% charge at month 12; it is 10% at 6 months and another 10% at 12 months. For serious cases where HMRC believes you've deliberately withheld information, the penalty can rise higher still.

Using £5,000 in unpaid corporation tax as an example:

Filing deadline before 1 April 2026:

• 1 day late: £100
• 3 months late: +£100 = £200 total in fixed penalties
• 6 months late: +£500 (10% of £5,000) = £700 total
• 12 months late: +£500 (further 10% of £5,000) = £1,200 total

Filing deadline on or after 1 April 2026:

• 1 day late: £200
• 3 months late: +£200 = £400 total in fixed penalties
• 6 months late: +£500 (10% of £5,000) = £900 total
• 12 months late: +£500 (further 10% of £5,000) = £1,400 total

And that's before interest charges.

Three Consecutive Late Returns: Higher Fixed Penalties

If your company has filed late for three consecutive accounting periods, HMRC levies higher fixed penalties for that third (and any later) late return. There are no daily penalties for CT600 late filing — the £10/day rule sometimes cited online applies to Self Assessment income tax returns, not Corporation Tax. The CT600 escalation is a higher fixed amount:

• Filing deadline before 1 April 2026: £500 initial + £500 at 3 months (instead of £100 + £100)
• Filing deadline on or after 1 April 2026: £1,000 initial + £1,000 at 3 months (instead of £200 + £200)

The percentage-based penalties at 6 and 12 months still apply on top.

CT600 Late Filing Penalty Summary Table

Running total on £5,000 unpaid tax (old regime): £100 → £200 → £700 → £1,200. Running total on £5,000 unpaid tax (new regime): £200 → £400 → £900 → £1,400.

Interest on Late Corporation Tax Payments

Penalties aren't the only cost. HMRC also charges interest on any corporation tax you haven't paid by the payment deadline (9 months and 1 day after your accounting period ends).

The current HMRC late payment interest rate is 7.75% per year (Bank of England base rate plus 4%, effective from 9 January 2026 — this rate changes when the base rate changes). Interest is charged from the day after the payment deadline until the day HMRC receives your payment.

Here's what that looks like in practice:

• £5,000 tax bill, 6 months late on payment: ~£194 in interest
• £10,000 tax bill, 12 months late on payment: ~£775 in interest

Interest is charged on top of penalties. Unlike penalties, you can't appeal interest charges — they're statutory and non-negotiable. One point worth knowing: late payment interest on corporation tax is deductible against your profits.

Need to understand interest rates in detail? See our guide on corporation tax interest: late payment rates and how to avoid charges.

What Happens Step by Step When You Miss the Deadline

Here's the typical sequence of events:

Week 1-2 After Deadline

• HMRC's system automatically generates the first fixed penalty
• You may receive a penalty notice through Government Gateway
• A letter is posted to your company's registered office

Month 1-3

• The penalty notice arrives by post (HMRC post can be slow)
• You may not even realise you've been penalised yet
• Your company's record at HMRC is flagged as having an outstanding return

Month 3

• Second fixed penalty is added automatically
• HMRC may send a further reminder
• If you also haven't paid your corporation tax, interest is accumulating daily

Month 6

• The 10% tax-gated penalty kicks in
• HMRC may issue a determination — their own estimate of your tax liability — and demand payment
• This estimate is often higher than your actual liability because HMRC doesn't have your figures

Month 12

• A further 10% tax-gated penalty is applied (20% total tax-geared across both stages)
• HMRC may take enforcement action (debt collection, county court judgements)
• Your company's credit rating could be affected

Beyond 12 Months

• HMRC can issue a notice to deliver under penalty
• In extreme cases, HMRC can apply to court for a winding-up order
• Your personal record as a director can be affected

Can You Appeal a CT600 Late Filing Penalty?

Yes — but only in specific circumstances. HMRC will consider an appeal if you had a reasonable excuse for filing late. Examples they accept include:

• Serious illness or hospitalisation (yours, not your accountant's)
• Death of a close family member around the deadline
• HMRC's own systems being down when you tried to file
• Fire, flood, or theft that destroyed your records
• Postal delays (if filing by paper, which is increasingly rare)

What HMRC does not accept as a reasonable excuse:

• "I didn't know I had to file"
• "My accountant didn't do it"
• "I was too busy running my business"
• "I didn't think I owed any tax"
• "I found the form confusing"

If you want to appeal, you need to write to HMRC within 30 days of the penalty notice, explaining your circumstances and providing evidence.

We've written a full guide on this: How to appeal a CT600 late filing penalty from HMRC.

How to Fix It: Filing Your CT600 Right Now

If you've missed your deadline, the single most important thing you can do is file as soon as possible. Every day counts.

Here's what to do:

Step 1: Gather Your Information

You need:

• Your company's accounts for the period
• Income and expenditure figures
• Any capital allowances or reliefs you're claiming
• Your company UTR (Unique Taxpayer Reference)
• Your HMRC Government Gateway login

Not sure what documents you need? Check our CT600 filing checklist.

Step 2: Calculate Your Corporation Tax

Use the Taxpipe calculator to estimate your corporation tax liability. You need to know:

• Your taxable profits
• Whether marginal relief applies (profits between £50,000 and £250,000)
• Any losses to carry forward

Step 3: File Your CT600

You have several options:

Option A: Use Taxpipe (fastest)

Taxpipe is built for exactly this situation. Answer a few questions about your company, upload or enter your figures, and we generate and file your CT600 with HMRC — including the iXBRL accounts that HMRC requires.

It takes most directors about 15 minutes and costs just £59. No accountant needed, no jargon, no waiting weeks for someone to get back to you.

Option B: Use an accountant

If your situation is complex (multiple income streams, R&D claims, group companies), an accountant may be worth the cost — but expect to pay £300-£1,500+ and wait days or weeks.

Option C: Other CT600 software

There are other options, but many are complex and expensive. See our CT600 software comparison for a full breakdown.

Step 4: Pay Any Corporation Tax Owed

File first, pay second. But do both as fast as possible. Interest accrues daily on unpaid tax.

Payment options include:

• Online banking (fastest — usually same or next day)
• BACS or CHAPS
• Direct debit
• At your bank or building society

Full payment guide: How to pay corporation tax to HMRC.

Step 5: Appeal Penalties (If You Have Grounds)

Once you've filed and paid, consider whether you have grounds to appeal any penalties. Don't let the appeal process delay your filing — file first, then appeal.

What If Your Company Made No Profit? You Still Need to File

One of the biggest misconceptions is that you don't need to file a CT600 if your company didn't make any money. Wrong. Every active company must file a CT600 return for every accounting period, even if:

• You made a loss
• You had zero income
• Your company was dormant for part of the period

The fixed penalties apply whether you owe £0 or £100,000. A nil return filed late still costs you at least £100 (or £200 for filing deadlines from April 2026) in penalties. The percentage-based charges at 6 and 12 months won't apply if your tax bill is zero — but the fixed penalties absolutely will.

Read more: Do I need to file a CT600 if my company made no profit?

Can a Company Be Struck Off for Not Filing a CT600?

Your CT600 is filed with HMRC — a separate matter from your statutory accounts at Companies House. Companies House can strike off companies that fail to file their annual confirmation statement and accounts. HMRC typically objects to any voluntary strike-off application while there are outstanding tax obligations, blocking the process until the position is resolved.

For persistent non-payment of corporation tax, HMRC has its own enforcement route: it can apply to court for a winding-up order under the Insolvency Act 1986. This is a more severe outcome than strike-off and results in formal liquidation.

For a single missed CT600 deadline these extreme outcomes are unlikely — but if you've missed multiple years of filings and ignored HMRC's correspondence, the risk grows. A company wound up or struck off loses its legal existence. Its assets — including any money in its bank account — pass to the Crown or liquidator. Getting it restored is expensive and time-consuming.

More on this: Company struck off or dissolved? You may still need to file a CT600.

Real-World Scenarios

Scenario 1: "I Just Forgot — It's 2 Weeks Late"

Damage: One fixed penalty, possibly some interest if you also haven't paid your tax.

• Filing deadline before 1 April 2026: £100 penalty
• Filing deadline on or after 1 April 2026: £200 penalty

Fix: File immediately. The first fixed penalty is already locked in, but you can stop it getting worse. If you file within 3 months of your deadline, you'll avoid the second fixed penalty and all the percentage-based penalties.

Cost with Taxpipe (new regime example): £59 for filing + £200 penalty = £259 total.

Scenario 2: "My Accountant Didn't File It — It's 4 Months Late"

Damage: Two fixed penalties, plus interest on unpaid tax.

• Filing deadline before 1 April 2026: £100 + £100 = £200 in fixed penalties
• Filing deadline on or after 1 April 2026: £200 + £200 = £400 in fixed penalties

Fix: File yourself using Taxpipe — don't wait for your accountant. You can always switch back later. The priority is stopping the penalty clock.

Cost with Taxpipe (new regime example): £59 for filing + £400 in penalties = £459 total (plus any interest on unpaid tax).

Scenario 3: "I Didn't Know I Had to File — It's 8 Months Late"

Damage: Two fixed penalties + 10% of your estimated tax liability.

• Filing deadline before 1 April 2026: £200 in fixed penalties + 10% of unpaid tax
• Filing deadline on or after 1 April 2026: £400 in fixed penalties + 10% of unpaid tax

Fix: File immediately and pay any tax owed. Then consider whether you have grounds to appeal. "I didn't know" isn't a valid excuse, but if there were genuine extenuating circumstances (you never received HMRC's notice to deliver, for example), it's worth trying.

How to Make Sure This Never Happens Again

1. Set calendar reminders — 3 months before your filing deadline, 1 month before, and 1 week before
2. File early — there's no benefit to waiting. You can file your CT600 as soon as your accounting period ends. The tax payment deadline stays the same regardless of when you file.
3. Use software that reminds you — Taxpipe sends deadline reminders so you never forget
4. Separate filing from payment — even if you can't afford to pay your tax bill yet, file the return on time. The filing penalties and payment penalties are separate. Filing on time avoids the filing penalties even if you're late paying.

Frequently Asked Questions

What is the penalty for filing CT600 1 day late?

The penalty for filing your CT600 just one day late is a fixed amount: £100 if your filing deadline was before 1 April 2026, or £200 for filing deadlines on or after that date. This automatic penalty applies regardless of how much tax your company owes — even if the answer is £0. The penalty is triggered the instant your filing deadline passes without a submitted return.

What happens if I've filed late for three consecutive periods?

If your company has filed its CT600 late for three consecutive accounting periods, the fixed initial penalties are significantly higher for that third (and any later) late return. For filing dates before 1 April 2026, the initial penalty rises from £100 to £500, and the three-month penalty from £100 to £500. For filing dates on or after 1 April 2026, the equivalent figures are £1,000 + £1,000. There are no daily penalties for CT600 late filing — that rule applies only to Self Assessment (personal income tax), not Corporation Tax.

Can I appeal a CT600 late filing penalty?

Yes, you can appeal if you have a reasonable excuse. HMRC accepts reasons like serious illness, bereavement, natural disasters, or HMRC system failures. They do not accept "I forgot," "I was busy," or "my accountant didn't do it." Appeals must be made within 30 days of the penalty notice. See our full appeal guide.

Do I get penalised even if my company owes no tax?

Yes. CT600 filing penalties are completely separate from your tax bill. Even if your company made no profit and owes £0 in corporation tax, you'll still be charged the first fixed penalty for filing one day late and the second fixed penalty at three months. The percentage-based penalties at 6 and 12 months won't apply if your tax bill is £0, but the fixed penalties absolutely will.

What's the difference between filing late and paying late?

Filing late means you haven't submitted your CT600 return to HMRC. Paying late means you haven't paid the corporation tax you owe. They have different deadlines (filing is 12 months after your year-end; payment is 9 months and 1 day) and different penalty structures. You can be penalised for one, both, or neither independently.

Can HMRC estimate my tax bill if I don't file?

Yes. After your filing deadline passes, HMRC can issue a determination — their estimate of your tax liability. This is legally binding until you submit your actual return. HMRC's estimates tend to be conservative (i.e., high), so it's in your interest to file and replace their estimate with your actual figures as soon as possible.

What happens if I'm late filing for multiple years?

Penalties stack. Each accounting period has its own set of penalties. If you're late for three or more consecutive periods, the fixed initial penalties escalate significantly — to £500 or £1,000 per stage depending on regime. Multiple years of late filing significantly increases HMRC's scrutiny of your company and may lead to formal enquiries.

Is there a way to file my CT600 quickly?

Yes. Taxpipe lets you file your CT600 in about 15 minutes for just £59. We handle the iXBRL accounts, the calculations, and the submission to HMRC. If you're already late, this is the fastest way to stop the penalty clock. Get started now →

Stop the Penalty Clock — File Today

If you're reading this because you've missed your deadline, here's the bottom line: every day you wait costs you more money. The penalties escalate, the interest accumulates, and HMRC's patience runs out.

Taxpipe makes filing your CT600 fast, simple, and affordable:

• ✅ £59 flat fee — no hidden costs
• ✅ 15-minute process — answer questions in plain English
• ✅ iXBRL included — we generate the accounts HMRC requires
• ✅ Direct HMRC submission — filed electronically, instantly
• ✅ Deadline reminders — so this never happens again

File your CT600 now → | See pricing → | Calculate your tax →

The first fixed penalty is already done — but you can still avoid the second charge, the 10%, and the further 10%. Act today.

Related Reading

• What Happens If You Don't File Corporation Tax? Penalties, Interest, and How to Fix It
• Corporation Tax Interest: Late Payment Rates, Repayment Interest & How to Avoid Charges
• Just Incorporated? Your First CT600 Timeline: Key Dates You Can't Miss