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.

Here's the full breakdown:

1 Day Late: £100 Penalty

The second your CT600 is overdue — even by a single day — HMRC issues an automatic £100 penalty. There's no human review. It's computer-generated.

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 £100 Penalty

If your CT600 still hasn't been filed after 3 months, HMRC adds a second penalty of £100. That's now £200 total in fixed penalties alone.

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 £200 you already owe.

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 set the record straight.

12 Months Late: 20% of Unpaid Tax

If a full year passes without filing, the penalty rises to 20% of the estimated unpaid tax. For serious cases where HMRC believes you've deliberately withheld information, this can be even higher.

Using the same £5,000 example:

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: +£1,000 (20% of £5,000) = £1,700 total

And that's before interest charges.

Daily Penalties (for Repeat Offenders)

If your company has filed late for three consecutive accounting periods, HMRC can impose additional daily penalties of £10 per day for the first 3 months after the initial deadline. That adds up to roughly £900 on top of everything else.

So if you're consistently late, the total penalty for the fourth (or later) late filing could look like this:

£100 (1 day late)
£900 (daily penalties for ~90 days)
£100 (3 months)
10% of tax (6 months)
20% of tax (12 months)

For a company owing £10,000 in tax, that's potentially £4,100 in penalties alone.

CT600 Late Filing Penalty Summary Table

When	Penalty	Running Total (£5k tax)
1 day late	£100 fixed	£100
3 months late	£100 fixed	£200
6 months late	10% of unpaid tax	£700
12 months late	20% of unpaid tax	£1,700
Daily (repeat offenders)	£10/day for ~90 days	+£900

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 7.25% per year (as of early 2025 — this rate changes with the Bank of England base rate). 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: ~£181 in interest
£10,000 tax bill, 12 months late on payment: ~£725 in interest

Interest is charged on top of penalties. And unlike penalties, you can't appeal interest charges — they're non-negotiable.

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 a £100 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 £100 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

20% tax-gated penalty applied
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 petition to strike off your company
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 — literally, if you're in daily penalty territory.

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 or back

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 same penalties apply whether you owe £0 or £100,000. A nil return filed late still costs you at least £100 in penalties.

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

Can HMRC Strike Off Your Company for Not Filing?

Yes. HMRC has the power to request that Companies House strike off your company if it consistently fails to file returns. This is relatively rare for a single missed return, but if you've missed multiple deadlines, it becomes a real risk.

A struck-off company loses its legal existence. Its assets — including any money in its bank account — pass to the Crown. 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: £100 penalty, possibly some interest if you also haven't paid your tax.

Fix: File immediately. The £100 penalty is already locked in, but you can stop it getting worse. If you file within 3 months, you'll avoid the second £100 penalty and all the percentage-based penalties.

Cost with Taxpipe: £59 for filing + £100 penalty = £159 total.

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

Damage: £200 in fixed penalties (£100 + £100), plus interest on unpaid tax.

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: £59 for filing + £200 in penalties = £259 total (plus any interest on unpaid tax).

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

Damage: £200 in fixed penalties + 10% of your estimated tax liability.

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

Set calendar reminders — 3 months before your filing deadline, 1 month before, and 1 week before
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.
Use software that reminds you — Taxpipe sends deadline reminders so you never forget
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 £100. This is an automatic, fixed penalty that HMRC 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.

How much are daily penalties for late CT600 filing?

Daily penalties of £10 per day apply for up to 90 days (approximately £900 total) — but only if your company has filed late for three or more consecutive accounting periods. These are additional to the standard £100 and £200 fixed penalties.

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 £100 for filing one day late and £200 total 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, daily penalties of £10/day also kick in. 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 £100 penalty is already done — but you can still avoid the £200, the 10%, and the 20%. Act today.