Why it matters
A company must file one when HMRC sends it a notice to deliver a Company Tax Return, even if it made a loss or has no tax to pay. It is due 12 months after the end of the accounting period it covers, and it has to be filed online, unless you have a reasonable excuse or are filing in Welsh3. A late return costs £200 straight away, and another £200 at 3 months2.
An accounting period for Corporation Tax can’t be longer than 12 months, so a company’s first, longer set of accounts often needs two CT600s. See When are company accounts due?, and HMRC’s free filing service has closed for how to file one now. Fileyear prepares and files it with your accounts and tax return.