ARTG4850 | Review of direct and indirect taxes decisions: Conclusion of the review: What happens if we do not complete the review within the time limit
From HM Revenue & Customs · Appeals reviews and tribunals guidance
If the review officer does not send a conclusion of review letter to the customer at the end of 45 days (or any agreed longer period) they should contact the customer, by phone if possible, and negotiate an extension to the review period. They should then write to the customer and confirm the agreed extended time limit for completion of the review.
If the customer does not agree to an extension of the review period, the review officer must stop work on the review and instead write to the customer to tell them that
the decision is treated as upheld, see ARTG4830, and
If the customer has an agent acting on their behalf, the review officer must write directly to the customer and send a copy of this letter to the agent.
If we have written to ask the customer proposing a longer review period and the customer agrees to the extension after the review period expired, the customer cannot appeal to the tribunal until the agreed longer review period expires or, if earlier, the review is completed.