The payment date usually comes before the CT600 filing date

For many small UK companies, Corporation Tax is due 9 months and 1 day after the end of the accounting period. The Company Tax Return filing deadline is normally later: 12 months after the end of the accounting period.

That gap is the trap. A company can still be inside the CT600 filing window while already owing late payment interest because the tax was not paid on time. Good CT600 software should therefore treat payment timing as part of the review workflow, not as an afterthought once the return is ready.

What to capture before filing

A filing workflow should make the deadline position obvious enough for a preparer, reviewer, director, or agent to understand.

the Corporation Tax accounting period covered by the CT600;
whether the accounts period creates one or more Corporation Tax accounting periods;
the normal payment deadline for the period;
payments already made to HMRC and the dates they were made;
late payment interest exposure where payment was after the due date;
the separate CT600 filing deadline and any late filing risk.

Payment evidence changes the workflow

The return file should not stop at the tax calculation. It should also show what payment evidence exists, who is responsible for payment, whether HMRC has allocated the payment to the right period, and whether any client follow-up is still open.

For accountancy practices, the practical issue is handoff. A clean small-company return can still create avoidable follow-up if the payment instruction, payment reference, approval trail, and HMRC account evidence are separated from the CT600 file.

How late payment interest fits into Corporation Tax work

HMRC charges interest where Corporation Tax is paid late. GOV.UK says HMRC interest rates are linked to the Bank of England base rate, with late payment interest set by the published formula. From 6 April 2025, late payment interest is set at base rate plus 4 percentage points.

Interest is not a penalty for filing the CT600 late. It is tied to payment timing. A company can have a clean filing process and still incur interest if the tax was paid after the due date.

HMRC also says late payment interest paid to HMRC is tax deductible for Corporation Tax purposes. That creates a follow-on accounting and tax-computation point for the period in which the interest is incurred.

Late filing penalties are a separate risk

Late payment interest should be reviewed separately from late filing penalties. GOV.UK's Company Tax Return penalty guidance explains what can happen where the return is late, including HMRC determinations where a return is more than 6 months late.

A useful practice workflow therefore needs two clocks: the money clock and the return clock. They interact commercially, but they are not the same obligation.

Common deadline traps

Assuming the accounts year is always one CT period: a long accounts period can create two Corporation Tax accounting periods and two returns.
Waiting for final accounts before thinking about payment: the payment deadline may arrive before every review point is settled.
Ignoring payment evidence: a filed return does not prove HMRC received or allocated the payment.
Treating submission evidence as payment evidence: Government Gateway filing status does not prove the Corporation Tax bill was paid.
Forgetting interest in the next computation: HMRC late payment interest can affect the accounts and tax computation for the period incurred.

How Robocount supports deadline review

Robocount is built around the CT600 filing workflow, but the review trail also needs to show whether the return is being filed before or after key payment dates. That matters for accountants explaining a file to a client and for directors trying to understand cash flow.

Connects the CT600 accounting period to filing and payment review points.
Keeps Corporation Tax liability, supplementary-page effects, and return status visible together.
Supports practice review notes for payment timing, late interest, and client approval.
Gives API and AI-assisted workflows structured dates instead of free-text deadline assumptions.
Preserves the distinction between filing evidence and payment evidence.

FAQ

When is Corporation Tax due for a small company?

For many small companies, GOV.UK says Corporation Tax is due 9 months and 1 day after the end of the accounting period. Check the company's facts and current HMRC guidance before relying on a payment date.

Is the CT600 due on the same day as the tax payment?

No. The Company Tax Return deadline is normally 12 months after the end of the accounting period, while payment for many small companies is due earlier.

Does filing the CT600 pay the Corporation Tax bill?

No. Filing the return and paying the tax are separate actions. The review file should keep submission evidence and payment evidence distinct.

Can late payment interest be included in the company's tax computation?

HMRC says late payment interest paid to HMRC is tax deductible for Corporation Tax purposes, so it can be included in the accounts for the period in which the interest was incurred.

Useful official references

This guide is general product and filing workflow information, not tax advice. Check current GOV.UK guidance and the company's exact facts before filing or paying.

Corporation Tax payment deadlines, CT600 filing timing, and interest.