Missing your Corporation Tax deadline is more common than you'd think — and the consequences escalate quickly. Whether you've missed by a day or a year, this guide explains exactly what happens, what penalties you'll face, and how to get back on track.

The Two Deadlines You Need to Know

Corporation Tax has two separate deadlines, and confusing them is one of the most common mistakes:

1. Payment Deadline: 9 Months + 1 Day After Period End

Corporation Tax must be paid within 9 months and 1 day after your accounting period ends. For a company with a 31 March 2026 year-end, payment is due by 1 January 2027.

Miss this deadline and HMRC charges interest on the unpaid amount from the day after the deadline. The current late payment interest rate is set by HMRC and changes periodically — check the latest rate on GOV.UK.

2. Filing Deadline: 12 Months After Period End

Your CT600 return must be filed within 12 months of your accounting period end. For the same 31 March 2026 year-end, filing is due by 31 March 2027.

Miss this deadline and HMRC charges flat penalties that increase over time, plus potential tax-based penalties for longer delays.

The Penalty Escalation Timeline

Day 1: £100 Fixed Penalty

The moment you miss the filing deadline — even by one day — HMRC charges a £100 penalty. This is automatic. No warning, no grace period.

3 Months Late: Another £100

If you still haven't filed after 3 months, HMRC adds another £100 penalty. You're now £200 down for a return that might show zero tax liability.

6 Months Late: HMRC Estimates Your Tax

After 6 months, things get serious. HMRC sends a tax determination — they estimate your tax bill based on whatever information they have (which is usually not in your favour). They then charge 10% of the unpaid tax shown on their determination as an additional penalty.

You cannot appeal this determination — the only way to override it is to actually file your CT600 return.

12 Months Late: Another 10%

After 12 months of non-filing, HMRC charges a further 10% of the unpaid tax as a penalty. If your tax determination was £5,000:

£100 (1 day late)
£100 (3 months late)
£500 (6 months — 10% of £5,000)
£500 (12 months — 10% of £5,000)
Total penalties: £1,200 — plus interest on all amounts

Beyond 12 Months: Potential Criminal Investigation

For persistent non-filers, HMRC can:

Issue daily penalties of up to £60 per day
Open a compliance investigation
In extreme cases, pursue criminal prosecution for fraudulent evasion
Strike off your company at Companies House

Interest on Late Payment

On top of penalties, HMRC charges interest on any unpaid Corporation Tax from the day after the payment deadline. Interest compounds, so the longer you leave it, the more it costs.

Interest is charged on:

The unpaid Corporation Tax itself
Any penalties that remain unpaid after 30 days

Unlike penalties, you cannot appeal interest charges. They're automatic and non-negotiable.

What About Dormant Companies?

Even if your company is dormant and has no tax to pay, you must still file a CT600 if HMRC has issued a notice to deliver. The £100 and £200 penalties apply regardless of whether any tax is owed.

Many directors assume that a dormant company doesn't need to file. This is wrong if:

HMRC has sent you a notice to deliver a return
Your company has any income, even bank interest
Your company has filed accounts showing it was active

If your company is genuinely dormant and you haven't received a notice to deliver, you may be able to inform HMRC that the company is dormant. But if a notice has been issued, you must file.

Can You Appeal a Late Filing Penalty?

Yes, but only in limited circumstances. HMRC accepts appeals based on reasonable excuse, which includes:

Serious illness of the director or their close relative
Death of the director or partner
HMRC system failures that prevented online filing
Fire, flood, or natural disaster affecting your records
Postal delays (for paper returns sent in good time)

HMRC does not accept these as reasonable excuses:

You didn't know the deadline
Your accountant let you down
You were too busy
You didn't have the funds to pay an accountant
You forgot

If you believe you have a reasonable excuse, appeal within 30 days of the penalty notice.

How to Fix It: Filing Late

The best course of action if you've missed your deadline is to file as soon as possible. Every day of delay potentially increases your penalties.

Step 1: Calculate Your Tax Position

Gather your financial records for the accounting period. Even if they're incomplete, file with your best estimates — you can amend the return later.

Step 2: File Your CT600

You can file online using HMRC-recognised software. You'll need:

Your Company Registration Number (CRN)
Your Unique Taxpayer Reference (UTR)
Your HMRC Gateway credentials
Your income and expense figures

Step 3: Pay Any Tax Owed

Pay as soon as possible to stop interest accruing. HMRC accepts payment via:

Bank transfer (Faster Payments, BACS, or CHAPS)
Direct Debit
Corporate credit/debit card (fees may apply)
At your bank or building society

Step 4: Contact HMRC if You Can't Pay

If you can't pay the full amount, contact HMRC's Payment Support Service on 0300 200 3835 to discuss a Time to Pay arrangement. Don't ignore the bill — HMRC is generally more lenient with businesses that communicate proactively.

Multiple Years of Unfiled Returns

If you have several years of unfiled CT600 returns, the penalties stack up for each year independently. However, HMRC sometimes shows leniency when you come forward voluntarily — this is called a voluntary disclosure.

Priorities for multiple unfiled returns:

File the most recent year first (stops new penalties accumulating)
Work backwards through previous years
Pay what you can immediately
Set up a Time to Pay arrangement for the remainder

Striking Off vs Filing

Some directors consider striking off their company to avoid filing obligations. Be aware that:

You must still file returns for all periods up to the strike-off date
HMRC can object to a strike-off if returns are outstanding
Directors can be held personally liable for company debts (including tax) if the company is struck off while owing money
A struck-off company can be restored to the register, and you'll still owe the tax

Striking off doesn't make the tax obligations disappear.

Prevention: Never Miss a Deadline Again

Set calendar reminders for both the 9-month payment deadline and 12-month filing deadline
File early — there's no benefit to waiting until the deadline
Use a filing deadline calculator — our CT600 deadline calculator can help
Keep records throughout the year — don't leave everything to year-end
Use software that tracks deadlines — modern filing tools can remind you

File Your Overdue CT600 Now

If you have an outstanding CT600, the best time to file was before the deadline. The second best time is right now.

Taxpipe can help you file your CT600 quickly and affordably — just £59 per return. Our guided wizard walks you through every step, computes your tax automatically, and submits directly to HMRC. The sooner you file, the sooner penalties stop accumulating.

Start Filing Now →

This article is for general information only and does not constitute tax advice. For complex situations involving multiple unfiled returns or potential investigations, consult a qualified tax adviser.

Related guides:

Corporation Tax Payment Reference — How to Pay HMRC Correctly

What Happens If You Don't File Corporation Tax? Penalties, Interest, and How to Fix It