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HMRC internal manual

VAT Refunds

Unjust enrichment: Reimbursement: Extension to the 90 day time limit

It is our view that, 90 days is a reasonable time for businesses to pass refunds to consumers, and we would normally expect businesses to have their scheme ready to implement by the time the signed undertaking is received. 

In certain cases it may not be possible for the claimant to make the refund within 90 days, for example, because he initially wrote to customers to check their addresses.

If 90 days is not long enough to refund all the money and you are satisfied that genuine efforts are being made to reimburse customers (eg many have already received their money) you can exercise your discretion and allow a slightly longer period - perhaps a month or two.