HMRC imposes a strict 2-year time limit for amending Corporation Tax Returns online. This deadline runs from the end of the statutory filing deadline for the accounting period in question, meaning you typically have 12 months from the filing deadline - which is itself 12 months after the end of your accounting period - to submit any amendments through the standard online process.

For example, if the accounting period ended on 31/12/2025, you have until the 31/12/2027 to submit an amendment online.

Once this 2-year window has passed, the return is considered final and amendments can no longer be submitted electronically. Any corrections required after this point would need to be made by post, by writing directly to HMRC with full details of the changes required and the reasons for the late amendment. HMRC will then review the request and determine whether any adjustments can be made, though there is no guarantee these will be accepted outside of the standard amendment window.

It is therefore strongly recommended to review your Corporation Tax return carefully and submit any necessary amendments well within the 2-year deadline to avoid this process.