CTM95600 - Corporation Tax self-assessment (CTSA): Discovery determination: CTPF: Direction - 20 year time limit
TMA70/S41C provides a normal time limit for CT directions of six years from the end of the accounting period to which the determined amount relates.
The time limit for directions is to protect companies against unreasonably late assessments.
You can make a direction at any time not later than 20 years from the end of the period to which the determination relates. You can only do this to correct an excessive determination that is the result of fraudulent or negligent conduct of the company (or a person acting on its behalf).