Corporation tax deadlines UK 2026
Nobody starts a limited company thinking about tax deadlines. Most directors are busy finding clients, growing revenue, or simply trying to keep the business running smoothly in the first year. Unfortunately, HMRC does not make allowances for busy schedules. Miss a Corporation Tax deadline and penalties can arrive quickly, even if the delay was accidental.
One of the biggest mistakes directors make is assuming there is only one Corporation Tax deadline. In reality, there are three separate deadlines, all with different dates and requirements. Mixing up the filing deadline with the payment deadline, or assuming Companies House filing covers HMRC obligations, can become an expensive error.
This guide explains every major Corporation Tax deadline for UK companies, how to calculate your own due dates, and the penalties that apply — including the increased late filing penalties introduced from April 2026.
If your CT600 filing deadline falls on or after 1 April 2026, the new penalty regime applies to your company.
THE THREE CORPORATION TAX DEADLINES EVERY DIRECTOR MUST KNOW
Many business owners refer to “the Corporation Tax deadline” as though it is one single date. It is not. UK limited companies face three separate obligations, and each comes with its own deadline.
| Obligation | Deadline |
|---|---|
| Pay Corporation Tax to HMRC | 9 months and 1 day after accounting period ends |
| File CT600 (Company Tax Return) with HMRC | 12 months after accounting period ends |
| File annual accounts with Companies House | 9 months after accounting period ends |
The important detail many directors overlook is this:
You must usually pay Corporation Tax before filing the CT600 return.
A common mistake is waiting for the accountant to finalise the return before paying HMRC. Unfortunately, HMRC still charges interest from the original payment deadline, regardless of when the return is submitted.
Filing annual accounts with Companies House is also completely separate from filing your CT600 with HMRC. Completing one does not satisfy the other obligation.
WHAT HAPPENS IF A DEADLINE FALLS ON A WEEKEND?
If your Corporation Tax payment deadline falls on a weekend or bank holiday, HMRC expects payment to arrive on the last working day before the deadline.
Bank transfers can take time to clear, and HMRC calculates late payment interest from the official due date rather than the date you sent the payment. Leaving it until the final evening can still result in interest charges.
HOW TO CALCULATE YOUR corporation tax deadlines UK 2026
Your Corporation Tax deadlines are based on the end date of your accounting period, which normally matches your company’s financial year end.
Example — Accounting Period Ends 31 March 2025
- Corporation Tax payment due: 1 January 2026
- CT600 filing deadline: 31 March 2026
- Companies House accounts due: 31 December 2025
For established companies, the process is fairly straightforward. However, newly incorporated businesses often face more complicated first-year filing requirements.
NEW COMPANIES MAY NEED TO FILE TWO CT600 RETURNS
When Companies House assigns your first accounting reference date, your initial financial year may run longer than twelve months.
While Companies House allows longer accounting periods for statutory accounts, HMRC does not apply the same rule to Corporation Tax.
A Corporation Tax accounting period can never exceed twelve months.
If your first financial year is longer than twelve months, HMRC automatically splits it into two separate Corporation Tax periods:
- The first period covers the first twelve months from the date the company became liable for Corporation Tax.
- The second period covers the remaining time up to your year end.
Each accounting period requires:
- A separate CT600 return
- Separate tax calculations
- Separate payment deadlines
This catches many first-time directors by surprise.
Large companies with profits above £1.5 million may also need to pay Corporation Tax through quarterly instalments rather than a single payment. The threshold is divided between associated companies, so growing businesses should review the rules carefully.
LATE FILING PENALTIES FOR CT600 RETURNS
Late filing penalties are issued automatically by HMRC. There is no warning period.
Even if your company owes no Corporation Tax at all, penalties still apply if the CT600 is submitted late.
The Government increased fixed late filing penalties from April 2026 for the first time in decades.
| How Late | Old Penalty (Before April 2026) | New Penalty (From April 2026) |
|---|---|---|
| 1 day late | £100 | £200 |
| 3 months late | Additional £100 | Additional £200 |
| 6 months late | 10% of unpaid tax | 10% of unpaid tax |
| 12 months late | Further 10% of unpaid tax | Further 10% of unpaid tax |
REPEAT LATE FILERS FACE HIGHER PENALTIES
Companies that submit returns late across three consecutive accounting periods face significantly higher fixed penalties.
Under the April 2026 rules:
- The initial fixed penalties can increase to £1,000 per missed milestone.
- Total fixed penalties alone may reach £2,000 before percentage-based tax penalties are added.
Repeated late filing quickly becomes extremely expensive.
HMRC TAX DETERMINATIONS AFTER SIX MONTHS
If six months pass without a CT600 being filed, HMRC may estimate your Corporation Tax bill and issue a tax determination.
This estimated figure is usually higher than the actual liability.
Important points directors should know:
- Tax determinations cannot be appealed.
- HMRC expects payment immediately.
- The only way to correct the amount is by filing the outstanding return.
Delaying further usually increases the financial damage.
LATE PAYMENT OF CORPORATION TAX
Paying Corporation Tax late does not trigger a fixed fine like late filing does.
Instead, HMRC charges daily interest from the day after payment was due until the balance clears in full.
From 9 January 2026, HMRC’s late payment interest rate is 7.75% per annum.
Example
- Corporation Tax owed: £30,000
- Payment due: 1 January 2026
- Actual payment date: 1 April 2026
Estimated interest:
£30,000 × 7.75% × (90 ÷ 365) = approximately £573
On larger tax liabilities, interest costs can rise very quickly.
PENALTIES FOR ERRORS IN CORPORATION TAX RETURNS
Submitting the CT600 on time does not protect you if the return contains mistakes that reduce the amount of tax owed.
HMRC may charge additional penalties depending on how the error occurred.
| Type of Error | Penalty Range |
|---|---|
| Careless error | 0% to 30% of additional tax |
| Deliberate but not concealed | 20% to 70% |
| Deliberate and concealed | 30% to 100% |
Companies that voluntarily disclose mistakes before HMRC discovers them usually receive reduced penalties.
This is known as an “unprompted disclosure.”
REGISTERING FOR CORPORATION TAX
Most limited companies are automatically registered for Corporation Tax after incorporation, and HMRC normally sends a Unique Taxpayer Reference (UTR) shortly afterwards.
However, directors should never assume registration happened correctly.
A company must notify HMRC within three months of starting business activity.
Failure to register can result in additional penalties based on unpaid tax.
You can check your registration status through your HMRC business tax account. If Corporation Tax does not appear in the account, contact HMRC directly.
APPEALING A CORPORATION TAX PENALTY
HMRC allows appeals against late filing penalties if the company had a reasonable excuse.
However, the threshold is strict.
You must usually file the outstanding return before submitting an appeal, and appeals must normally be made within 30 days of the penalty notice.
Situations HMRC May Accept
- Serious illness
- Bereavement
- HMRC system failures
- Fire, flood, or destruction of records beyond your control
Situations HMRC Usually Rejects
- Accountant delays
- Forgetting the deadline
- Assuming the company was dormant
- Cash flow problems
Although HMRC may consider payment arrangements for tax debts, these do not excuse late filing.
THE JOINT HMRC AND COMPANIES HOUSE FILING SERVICE HAS CLOSED
From 1 April 2026, the joint online filing service for submitting annual accounts and Company Tax Returns together permanently closed.
Companies must now:
- File CT600 returns using commercial accounting software or an accountant
- File annual accounts separately with Companies House
If you need historic returns from the old system, HMRC must be contacted directly.
FREQUENTLY ASKED QUESTIONS
WHAT IS THE CORPORATION TAX PAYMENT DEADLINE?
Corporation Tax must usually be paid 9 months and 1 day after the accounting period ends.
For example, if your year end is 31 March 2025, payment is due by 1 January 2026.
IS THERE A PENALTY FOR PAYING CORPORATION TAX LATE?
There is no fixed fine for late payment, but HMRC charges daily interest at 7.75% per annum from January 2026.
Even short delays can become costly on larger tax liabilities.
WHAT HAPPENS IF I DO NOT FILE A CT600?
Penalties increase automatically over time.
From April 2026:
- £200 after one day
- £400 after three months
- Additional tax-based penalties after six and twelve months
HMRC may also issue a tax determination after six months.
DOES A DORMANT COMPANY STILL NEED TO FILE?
Yes. Dormant companies can still face late filing penalties if the CT600 deadline is missed.
CAN I APPEAL A CORPORATION TAX PENALTY?
Yes, but you must normally show a reasonable excuse such as illness, bereavement, or HMRC technical failures.
The outstanding return should usually be filed before appealing.
HOW DO I KNOW IF MY COMPANY IS REGISTERED FOR CORPORATION TAX?
Log into your HMRC business tax account.
If Corporation Tax appears within the account, your company is registered.
SUMMARY OF THE MOST IMPORTANT CORPORATION TAX DEADLINES
UK companies face three separate Corporation Tax deadlines:
- Corporation Tax payment: 9 months and 1 day after the accounting period ends
- Companies House accounts: 9 months after the accounting period ends
- CT600 filing deadline: 12 months after the accounting period ends
The payment deadline arrives before the filing deadline, which is where many directors make mistakes.
From April 2026, HMRC doubled the fixed penalties for late filing:
- £200 after one day
- £400 after three months
Repeat late filers may face penalties of up to £1,000 per missed milestone.
Late payment interest at 7.75% per annum can also become expensive quickly on larger tax balances.
If your company is already behind, filing the return as soon as possible helps stop additional penalties from escalating further. In many cases, professional accounting support costs far less than the penalties generated by missed deadlines.