There is not a universal deadline for filing your CT600 corporation tax return; instead the date is unique to your company. You must file your return within 12 months of the end of the accounting period the return covers. This is also known as the ‘statutory filing date’. If you miss this deadline then HMRC will issue your company with a fine.
The deadline for filing your tax return is not the same as the deadline for paying any corporation tax which is due. Instead this must be paid within 9 months and one day following the end of the accounting period for those businesses with taxable profits under £1.5 million. This means you could in theory pay your corporation tax bill before filing the associated tax return; however, it is often the case that both of these things are done at the same time. Should your taxable profits be over £1.5 million then you will need to pay your corporation tax in four instalments; contact HMRC for further information on how you should proceed. Failure to pay your corporation tax bill on time will likewise result in a fine from HMRC.