United Kingdom / Guides / Corporation tax: the CT600 and the two different deadlines
United Kingdom · guideCorporation tax: the CT600 and the two different deadlines
The short answer
UK Corporation Tax runs on two separate clocks tied to the same accounting period. Payment is due 9 months and 1 day after the accounting period ends; the CT600 return reporting that liability is due 12 months after the same period ends, about three months later. That ordering means a company has to calculate and pay its own estimated tax bill before HMRC has seen the return confirming the figure, and interest accrues from the payment deadline regardless of whether the return itself is filed on time. Companies with taxable profits above £1.5 million follow a different, instalment-based payment schedule instead of this single date.
Key facts — verified dates on each
Two deadlines, not one, and they run in the wrong order
Most UK companies assume a return is filed first and a tax bill follows from it. Corporation Tax works the other way round. For a company with taxable profits up to £1.5 million, the tax itself is due 9 months and 1 day after the end of the accounting period it relates to. The Company Tax Return — the CT600, along with the accounts and tax computations filed alongside it — is not due until 12 months after the end of the same accounting period. The payment deadline lands roughly three months before the filing deadline, not after it.
That gap exists because Corporation Tax is a self-assessment tax: the company calculates its own liability and pays it without waiting for HMRC to issue an assessment first. The CT600 filed three months later is the formal record that confirms — or corrects — a figure the company has already worked out and paid on its own estimate.
How the accounting period sets both clocks
Both deadlines are measured from the company's accounting period for Corporation Tax purposes, which is not automatically the same thing as its full financial year. An accounting period cannot exceed 12 months. A company whose first financial year at Companies House runs longer than 12 months — an 18-month first year is common — is split by HMRC into two separate Corporation Tax accounting periods, and each one carries its own independent 9-months-and-1-day payment date and 12-month filing deadline.
For a straightforward 12-month accounting period ending 31 March, payment is due by 1 January the following year, and the CT600 is due by 31 March the year after that — two different calendar dates, both triggered by the same period-end.
Filing late: the penalty schedule
HMRC applies a fixed, escalating schedule of penalties when the CT600 itself is filed after the 12-month deadline, independent of whether the tax was paid on time.
If a return is late three times in a row, the £200 penalties at each of those two stages rise to £1,000 each. At the 6-month mark, HMRC also issues a "tax determination" — its own estimate of what the company owes — which cannot be appealed even though the late filing penalty itself can be, where the company has a reasonable excuse.
- 1 day late: a £200 penalty
- 3 months late: another £200 penalty
- 6 months late: HMRC estimates the tax bill and adds a penalty of 10% of the unpaid tax
- 12 months late: another 10% of any unpaid tax
Paying late: interest, not a fixed penalty
Late payment is handled separately from late filing. Rather than a fixed fine, HMRC charges late payment interest daily on the outstanding balance, from the day after the 9-months-and-1-day due date until the tax is paid in full. The rate is set at the Bank of England base rate plus 4 percentage points and moves whenever the base rate does — it is a formula, not a number to memorise, and the current published rate should be checked at the time it matters rather than assumed.
Because interest tracks the payment deadline and penalties track the filing deadline, it is entirely possible to file the CT600 on time at the 12-month mark and still owe interest, if the amount paid three months earlier under-estimated the final liability. HMRC also pays credit interest, at a lower rate, on Corporation Tax paid early.
Companies with profits over £1.5 million pay differently
The single 9-months-and-1-day payment date applies to companies with taxable profits up to £1.5 million. Above that threshold, HMRC requires payment in quarterly instalments instead of one lump sum, with the schedule differing again for companies with profits above £20 million. Most early-stage and small companies fall under the £1.5 million threshold and use the single payment date described above, but the instalment regime is worth knowing exists once profits approach that level.
Who prepares and files the CT600
The CT600, together with the statutory accounts and the tax computations it is filed with, is normally prepared and submitted by the company's accountant or tax adviser, using HMRC-recognised software or the joint filing arrangement that also satisfies Companies House. Because the payment falls due three months before the return does, the person managing the calculation typically has to produce a working estimate of the liability well ahead of finalising the return itself — the estimate that gets paid at the 9-months-and-1-day mark is not a formality, since interest starts accruing on any shortfall in it immediately after that date passes.
CapEasy serves accounting and compliance clients in the United States and Australia today. UK company services, including Corporation Tax and CT600 filings, are under consideration but not currently offered; this guide is provided for orientation only.
The figures, and when we checked them
These numbers change by year or by notification. Each one shows the date we last verified it against the source — if that date looks old, check the source before relying on it.
Questions on this
When is Corporation Tax actually due for payment?
For a company with taxable profits up to £1.5 million, 9 months and 1 day after the end of the accounting period the tax relates to. Companies above that threshold pay in instalments on a different schedule instead.
When is the CT600 Company Tax Return due?
12 months after the end of the same accounting period — about three months after the payment deadline, not before it.
Why is the payment deadline before the filing deadline?
Corporation Tax is a self-assessment tax: the company calculates and pays its own estimated liability without waiting for HMRC to assess it first. The CT600 filed three months later documents and confirms that figure rather than triggering the payment.
What happens if the amount paid at 9 months and 1 day turns out to be wrong?
If it was too low, interest accrues daily on the shortfall from the day after the payment deadline until it is paid, even if the CT600 itself is later filed on time. If it was too high, HMRC pays credit interest on the overpayment at a lower rate.
Does filing the CT600 on time avoid interest if the tax was paid late?
No. Interest is tied to the 9-months-and-1-day payment deadline, not the 12-month filing deadline, so the two run independently of each other.
What are the penalties for filing the CT600 late?
A £200 penalty applies from 1 day late, another £200 from 3 months late, then HMRC adds a penalty of 10% of the unpaid tax at 6 months late (based on its own estimate) and another 10% at 12 months late.
What happens if a company files late three times in a row?
The £200 penalties that apply at the 1-day and 3-month stages each increase to £1,000.
What is a "tax determination" and can it be appealed?
It is HMRC's own estimate of the tax owed, issued once a return is 6 months late. The determination itself cannot be appealed, though the late filing penalty can be, where the company has a reasonable excuse.
What is the current HMRC interest rate on late-paid Corporation Tax?
It was 7.75% from 9 January 2026, set at the Bank of England base rate plus 4 percentage points. Because it moves with the base rate, the current published figure should be checked directly rather than assumed from a past date.
Can a Corporation Tax accounting period run longer than 12 months?
No. An accounting period is capped at 12 months. A company whose financial year at Companies House is longer than that — an 18-month first year, for example — has that period split by HMRC into two separate accounting periods, each with its own 9-months-and-1-day payment date and 12-month filing deadline.
Primary sources
- GOV.UK — Pay your Corporation Tax bill
- GOV.UK — Company Tax Returns
- GOV.UK — Company Tax Returns: penalties for late filing
- GOV.UK — Rates and allowances: HMRC interest rates for late and early payments
Last reviewed 2026-08-14. Statutes and schedules change — the sources above are authoritative, this page is orientation.
This page is information, not an offer of services. CapEasy serves the US and Australia today; UK services are under consideration.