HMRC internal manual

Company Taxation Manual

Corporation Tax self-assessment (CTSA): the payment obligation: Due dates

You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.

CT is normally due and payable nine months and one day from the end of an accounting period.  There are examples at CTM01800, which reflect the legal position under CTSA and CT Pay & File.

For earlier accounting periods the extra day beyond nine months was allowed by concession.