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HMRC tax determination: what to do if you did not file a CT600

Last updated: 10 August 2026

Short version. A tax determination is HMRC's estimate of the Corporation Tax it thinks your company owes because a required CT600 has not been filed. It is not a substitute for the actual return. Act promptly to file the real return and deal with the tax and any penalties.

1. What it means

HMRC can issue a determination when a Company Tax Return remains unfiled. The figure is an estimate, so it may not match the company's true tax position. GOV.UK explains that a determination cannot be appealed on the amount itself. The practical way to replace it is to deliver the actual Company Tax Return.

Do not ignore the letter. It may come with late-filing penalties and interest, and delays can increase the amount HMRC asks the company to pay.

2. What to do now

  1. Identify the accounting period and the return HMRC says is outstanding.
  2. Check whether the company actually received a notice to deliver a CT600 for that period.
  3. Prepare and file the accurate return, including the required accounts and tax computation.
  4. Pay the tax due, or contact HMRC about payment if the company cannot pay in full.
  5. Keep the confirmation and documents after filing.

3. Appeals and late-filing penalties

You may be able to appeal a late-filing penalty if there is a reasonable excuse, but HMRC says you should file the return first. Use the information on the penalty notice and GOV.UK's official process rather than relying on a general article for a decision that depends on your evidence.

See HMRC's tax determination guidance and its current late-filing penalties for the authoritative rules.

4. Using SimpleCompanyTax

If the period is within our supported dates and the company is otherwise eligible, you can prepare the overdue return in SimpleCompanyTax. Use the actual period dates from HMRC's notice. A complex, very old or unsupported case needs an accountant.

Read which companies and periods we support before you begin. Filing through us does not remove an existing penalty, but it gives you the accurate return needed to resolve the outstanding filing.