Quick Answer
HMRC doubled its corporation tax late filing penalties from 1 April 2026. A CT600 filed one day late now triggers a £200 fixed penalty — up from £100. The Companies House late filing regime (separate from HMRC) is unchanged but runs alongside the HMRC penalties, meaning a director who misses both deadlines can face fines from two bodies simultaneously.
Corporation Tax Late Filing Penalties Doubled from 1 April 2026
HMRC increased its fixed penalties for late corporation tax returns (CT600) from 1 April 2026 — the first change to the penalty structure since 1998. Returns with a filing deadline on or after 1 April 2026 fall under the new rates. Returns due before that date are still assessed under the old rates even if filed late after the transition.
The official reason for the increase is straightforward: the fixed penalties had not changed in nearly three decades. In real terms, a £100 fixed penalty in 1998 is worth approximately half as much today — reducing the deterrent effect significantly. HMRC projects the increase will generate an additional £45m in 2026/27, rising to £70m annually by 2030/31.
The full penalty schedule is published on GOV.UK: penalties for late CT600 filing.
The New CT600 Penalty Schedule from April 2026
The penalties operate in two layers: fixed penalties that apply automatically to any late filing, and tax-geared penalties that apply only when corporation tax remains unpaid.
Fixed penalties for late CT600 filing
The fixed penalties below apply to any return with a filing deadline on or after 1 April 2026:
- Filed 1 day late: £200 (previously £100)
- Filed more than 3 months late: an additional £200 — total £400 (previously an additional £100, total £200)
- Three successive late filings (any): £1,000 per return (previously £500)
- Three successive late filings, each more than 3 months late: £2,000 per return (previously £1,000)
Fixed penalties apply even if the company owes no corporation tax — dormant companies, loss-making companies, and nil-return companies are all included.
Tax-geared penalties for unpaid corporation tax
Where corporation tax remains unpaid, HMRC adds percentage-based penalties on top of the fixed ones. These rates are unchanged from the pre-2026 regime:
- Return 6 months overdue: 10% of the unpaid corporation tax
- Return 12 months overdue: a further 10% of the unpaid corporation tax
If HMRC issues a tax determination (an estimated assessment after six months of non-filing), the percentage penalties are calculated against the determination figure, not the actual tax owed. The only way to override a determination is to file the actual CT600 return.
CT600 Late Filing Penalty — Day 1
CT600 Late Filing Penalty — 3 Months
Companies House Late Filing Penalty — 6+ Months (Private Co.)
Corporation Tax Payment Deadline
The CT600 Filing Deadline vs the Payment Deadline
One of the most common mistakes agency directors make is conflating the filing deadline with the payment deadline — they are different by three months.
- Corporation tax payment deadline: 9 months and 1 day after the end of your accounting period. If your year-end is 31 March 2026, payment is due by 1 January 2027.
- CT600 filing deadline: 12 months after the end of your accounting period. For a 31 March 2026 year-end, the return is due by 31 March 2027.
The practical consequence is that your corporation tax is due three months before you are required to file the return that calculates it. Directors who pay on the filing deadline rather than the payment deadline will owe interest on late payment even if they file on time. HMRC charges interest on the unpaid tax from the day after the payment deadline — currently at a rate linked to the Bank of England base rate plus 2.5 percentage points.
Using the director salary and dividend calculator can help you plan your personal tax position for the year — understanding your total corporation tax liability early gives you time to set aside the right amount before the payment deadline arrives.
Companies House Annual Accounts: A Separate Penalty Regime
The CT600 penalty regime and the Companies House annual accounts penalty regime are entirely separate. Missing your year-end can trigger fines from both bodies simultaneously.
For private limited companies, the Companies House late filing penalties are:
- Up to 1 month late: £150
- 1 to 3 months late: £375
- 3 to 6 months late: £750
- More than 6 months late: £1,500
All of these penalties double if accounts are filed late for two consecutive years. A company that files six months late twice in a row faces a £3,000 Companies House penalty on the second occasion — before any HMRC fines are counted.
Companies House penalties are automatic and issued without warning on the day after the deadline. The Companies House accounts deadline for a private limited company is 9 months after the accounting reference date (year-end). Full details of the penalty schedule are at GOV.UK: Companies House late filing penalties.
Why the iXBRL Mandate Compounds the Late Filing Risk
Most articles covering the April 2026 penalty increase treat it as a separate story from the iXBRL mandatory filing change that took effect on the same date. For UK limited company directors, they combine into a more significant risk than either change creates alone.
From 1 April 2026, all CT600 returns must be submitted in iXBRL format via commercial software. HMRC closed its free CATO filing service on 31 March 2026. A submission that is rejected by HMRC's Business Validation Rules due to incorrect iXBRL tagging does not count as filed — even if it was sent before the deadline. If the error is not identified and the corrected return resubmitted before the deadline, the filing counts as late and the £200 day-one penalty applies automatically.
The double jeopardy: doubled penalties, new software, same deadline
Companies transitioning to iXBRL software for the first time face the highest rejection risk — incorrect tagging, mismatched taxonomy selections, or missing computation data can all trigger a gateway rejection. This risk is highest at the moment the fixed penalty for a failed filing doubled. Setting up your new software at least two months before your next deadline — not in the week the return is due — is the only reliable safeguard.
Our full guide to the iXBRL transition covers the practical steps in more detail: iXBRL accounts filing 2026: what every UK limited company director must do now.
What to Do If You Have Already Missed a Deadline
Filing late is always better than not filing at all. Every additional month that passes without a CT600 filed increases the penalties — and after six months, HMRC can issue a tax determination, which shifts the burden of proof and adds interest from the determination date.
- File immediately. The moment you realise the deadline has passed, file the return. The penalty clock stops accumulating from the date of filing, not the date you decided to act.
- Pay any outstanding tax. If corporation tax is owed, pay it alongside the return to stop interest accruing further. HMRC's late payment interest rate currently sits at base rate plus 2.5 percentage points.
- Consider appealing if you have a reasonable excuse. Serious illness, bereavement, or a verifiable technical failure in HMRC's systems may support a successful appeal. Appeals must be submitted within 30 days of the penalty notice.
- Never ignore an HMRC tax determination. If you receive one, file the actual return immediately. Once filed, the determination is replaced by the real tax liability. Allowing the determination to stand means paying interest on HMRC's estimate, which is often conservative and may significantly overstate your actual liability.
Avoiding Corporation Tax Late Filing Penalties: A Director's Checklist
For UK agency directors managing multiple compliance deadlines across a financial year, late filing penalties are almost always preventable. The most common cause is not complexity — it is simply losing track of what is due and when.
- Know your year-end and the three dates it creates: the Companies House accounts deadline (9 months after year-end), the corporation tax payment deadline (9 months and 1 day after year-end), and the CT600 filing deadline (12 months after year-end)
- Confirm your iXBRL software is set up and tested well before your next filing — not the week before the deadline
- If you use an accountant, confirm they are using iXBRL-compliant software and have your records in time to file before the deadline
- Set calendar reminders at least 60 days before each deadline — both the payment deadline and the filing deadline, as they fall at different times
- Do not confuse dormant company obligations with nil returns. A dormant company still has a filing obligation and still faces the £200 penalty if it files a nil-return CT600 late
- If you have a group structure (holding company plus trading company, or multiple entities), each company has its own deadlines — they run independently
For broader compliance planning across the full financial year, our annual tax review checklist for directors covers corporation tax, VAT, PAYE, and Companies House obligations in one place. If you are considering changing accountants — perhaps because your current practice is not on top of these deadlines — our guide to switching accountants in the UK explains the process, including professional clearance letters and mid-year handovers.
The penalty increase is also a useful prompt to review whether your overall tax structure is working efficiently. Our guide to the optimal director salary and dividend mix for 2026/27 covers how to minimise your personal tax while meeting your corporation tax obligations.
How Alto Can Help
Alto Accounting is an ACCA registered practice specialising in UK digital and creative agencies. We manage CT600 filing, iXBRL accounts preparation, Companies House accounts submission, and the full year-end compliance cycle for agency founders and directors. If you are approaching a deadline and are not confident your return will be filed correctly and on time, we can review the position quickly.
The penalty increase from April 2026 is also a reasonable moment to check whether your current accountant is on top of the iXBRL transition. ACCA-registered practices are required to maintain compliant filing procedures as part of their professional obligations. You can confirm your accountant meets the new HMRC standards under the mandatory tax adviser registration scheme that launched in May 2026.
Book a free consultation to speak with a specialist.
Frequently Asked Questions
What is the penalty for filing a corporation tax return late in the UK?
From 1 April 2026, HMRC charges £200 for a CT600 filed one day late and a further £200 if it remains unfiled after three months — a total of £400. If a company has filed late in three successive years, each fixed penalty rises to £1,000 (or £2,000 if the return is more than three months late). On top of the fixed penalties, HMRC adds a tax-geared penalty of 10% of any unpaid corporation tax once the return is six months overdue, and a further 10% if it reaches 12 months. These rates doubled from 1 April 2026 — the first change since 1998.
Do late filing penalties apply if my company owes no corporation tax?
Yes. The fixed penalties for a late CT600 apply regardless of whether any corporation tax is owed. A dormant company, a loss-making company, or a company with a nil return still faces the £200 fixed penalty on day one and a further £200 after three months. The tax-geared penalties (10% at six months, 10% at twelve months) only apply if tax is actually unpaid, but the fixed penalties are automatic for any late submission.
What are the Companies House late filing penalties for private limited companies in 2026?
Companies House charges automatic penalties for late annual accounts based on how late the filing is. For private companies: up to one month late = £150; one to three months late = £375; three to six months late = £750; more than six months late = £1,500. All of these penalties double if accounts are filed late for two consecutive years. The penalties are issued automatically without warning on the day after the deadline.
Can I appeal a corporation tax late filing penalty?
You can appeal to HMRC if you have a reasonable excuse. Reasonable excuses HMRC typically accepts include a serious illness or bereavement affecting the person responsible for filing, a fire or flood that destroyed records, or a technical failure in HMRC's own systems. Poor administration, relying on a third party to file, or simply not knowing the deadline are generally not accepted. You must appeal within 30 days of receiving the penalty notice. If HMRC issues a tax determination (an estimated tax bill after six months of non-filing), the only way to challenge it is to file the actual CT600 return.
How does the iXBRL requirement from April 2026 affect late filing risk?
From 1 April 2026, all CT600 returns and accompanying accounts must be filed in iXBRL format using commercial software. HMRC closed its free CATO filing service on 31 March 2026. A submission that is rejected by HMRC's Business Validation Rules due to incorrect iXBRL tagging does not count as filed — even if it was submitted before the deadline. If the error is not corrected and the return resubmitted before the deadline, the filing counts as late and the penalty clock starts. Setting up new software well before the deadline, not the week it is due, is the practical safeguard.
What is an HMRC tax determination and how do I reverse it?
If a corporation tax return remains unfiled six months after the deadline, HMRC may issue a formal determination estimating the tax due. The determination becomes treated as the final tax liability, and interest on the estimated tax accrues immediately. Unlike a penalty notice, you cannot appeal the determination figure — the only way to override it is to file the actual CT600 return. Once filed, HMRC replaces the determination with the real tax liability. Filing late is always better than not filing at all.