For most limited companies the tax is due nine months and one day after the end of the accounting period.

A 31 March year end means payment by 1 January. A 30 June year end means 1 April. A 31 December year end means 1 October.

The return is a different date. Your CT600 is due twelve months after the period end, a full three months after the money.

That gap is where the trouble starts. Owners see the filing date in their diary and assume it is the deadline for everything.

One more trap. If your first accounting period runs longer than twelve months, it splits into two periods for tax. Each gets its own payment date, and the second one comes round quickly.

Payment has to clear by the deadline, so a Friday bank transfer on the due date can still land late. Check how long your chosen method takes on GOV.UK.

There is no fixed fine for paying corporation tax late. Interest is the cost, and it runs daily.

HMRC charged late payment interest at 7.75% from 9 January 2026. Since 6 April 2025 the rate has been the Bank of England base rate plus four per cent. It moves when the base rate moves, so check the current figure before you assume.

Repayment interest, paid to you when the company has overpaid, was 2.75% on the same date. That is base rate minus one per cent, with a floor of 0.5%.

The gap between the two rates is deliberate. Paying HMRC late is a more expensive way to borrow than most overdrafts, and leaving money with HMRC early earns you very little.

Interest is not a penalty you can appeal on reasonable excuse grounds. It compensates HMRC for the delay, so it stands even when the reason for the delay was fair.

Current rates sit on the HMRC interest rates page.

Filing late is judged separately from paying late, and the fixed penalties have just gone up.

For returns with a filing date on or after 1 April 2026, the first penalty is £200 rather than £100. More than three months late is £400 rather than £200.

Miss the deadline three times in a row and those figures become £1,000 and £2,000. The old amounts were set in 1998 and had been quietly eroded by inflation, so the government restored their real value at Budget 2025.

Tax geared penalties sit on top of the fixed ones. Once a return is eighteen months late, HMRC adds ten per cent of the unpaid tax. Another ten per cent follows at twenty four months.

So the worst outcome is a late return on top of unpaid tax, with interest running underneath both.

If you think a penalty is wrong, HMRC now has an interactive online form for corporation tax penalty appeals, which is faster than a letter. You still need a reasonable excuse, and being busy is not one.

Most owner managed companies pay in one lump. Instalments begin when taxable profits pass an annual rate of £1.5 million.

Above that figure and below £20 million, the tax is paid electronically in four instalments. Two of them fall before the year end, which is the part that surprises people. Companies above £20 million pay on a tighter schedule again.

Two exceptions are worth knowing. You stay outside instalments if the total liability for the period is under £10,000. You are also outside them where profits are no more than £10 million and the company either did not exist in the previous twelve months or had profits no higher than £1.5 million then.

Associated companies change the arithmetic. Both thresholds are divided by the number of associated companies plus your own. Two companies under common control halve the £1.5 million line for each of them.

If your profits are heading anywhere near that level, work the dates out a year early. The first instalment can be due six months and thirteen days into the accounting period, long before anyone has looked at the accounts. The rules are on GOV.UK.

Speak to HMRC before the deadline rather than after it.

A Time to Pay arrangement spreads the bill over agreed monthly payments. Interest still runs on the balance, but you avoid enforcement action and the conversation goes better while the date is still ahead of you. Start at the difficulties paying HMRC page.

File the return on time regardless of what the bank balance says. The fixed penalties are avoidable costs, and filing late tells HMRC something about the company that you would rather it did not think.

Then deal with the cause. In almost every late payment we have seen, the money existed during the year. It went out as dividends, or into a van, or into a hire, because nobody had put a figure on the tax yet.

Every Nexus client gets the corporation tax figure and their dividend headroom a month before the bill lands. There is nothing clever in it. It just means the cash is set aside while there is still time to set it aside.