Corporation Tax Payment Deadlines — What Small Companies Need to Know

Two deadlines, not one

One of the most common points of confusion for new company directors is that the deadline to pay Corporation Tax is not the same as the deadline to file the Corporation Tax return (the CT600). They are calculated differently, and the payment deadline actually falls before the filing deadline — which catches people out if they assume filing comes first.

When Corporation Tax is due for payment

For most small and medium-sized companies, Corporation Tax is due 9 months and 1 day after the end of the accounting period it relates to. For example, a company with an accounting period ending 31 March 2026 must pay its Corporation Tax by 1 January 2027.

This applies whether or not the CT600 return has been filed yet — HMRC expects payment based on your own estimate of the liability, calculated from your accounts, even if the formal return isn't due for a few more months.

When the CT600 return is due for filing

The Corporation Tax return itself (form CT600) is due 12 months after the end of the accounting period. Using the same example, a period ending 31 March 2026 has a CT600 filing deadline of 31 March 2027 — three months after the tax itself was due to be paid.

In practice, most accountants prepare the accounts and the CT600 well before the payment deadline, so the figure being paid is the actual calculated liability rather than an estimate. But the rules technically allow payment to come first.

Large companies pay differently

Companies with profits above certain thresholds are required to pay Corporation Tax in quarterly instalments during the accounting period, rather than as a single payment afterwards. This instalment regime is aimed at larger companies and does not apply to the vast majority of small UK companies caught by the small profits rate or marginal relief band — but if your company's profits are growing quickly, it is worth checking with your accountant whether you are approaching the threshold where instalment payments start to apply.

What happens if you pay late

If Corporation Tax is not paid by the due date, HMRC charges interest on the outstanding amount from the due date until it is paid, at HMRC's official late payment interest rate. This rate is reviewed periodically and can change, so check the current rate on gov.uk rather than assuming a figure from a previous year.

Filing the CT600 late is treated separately and attracts its own flat penalties that increase the longer the return remains outstanding, and escalate further for persistent lateness across consecutive periods. Paying late and filing late are two different compliance failures, each with its own consequence — it is possible to be on time with one and late with the other.

First accounting periods and long periods

A new company's first accounting period sometimes runs longer than 12 months, depending on the dates chosen at incorporation. HMRC does not allow a Corporation Tax accounting period to exceed 12 months, so a long first period is automatically split into two separate accounting periods for Corporation Tax purposes — each with its own payment and filing deadlines calculated from the dates above. This is a common source of confusion in year one and worth clarifying with your accountant at the very start, rather than after the first deadline has already passed.

Why this matters for your bookkeeping

The 9-months-and-1-day payment deadline arrives sooner than many new directors expect, especially when it falls in the middle of a busy trading period. Up-to-date monthly bookkeeping and management accounts mean you (and your accountant) know roughly what the Corporation Tax liability will be well before the year end closes — giving you time to set money aside, rather than discovering the figure only once the accounts are finalised, close to the payment deadline itself.

Note: this article explains how Corporation Tax deadlines generally work. QaisYasir Accounting Services' current service scope is bookkeeping, VAT/MTD, and management accounts — for your actual Corporation Tax computation, CT600 filing, and payment planning, work with a qualified accountant.