HMRC has said it will be writing to around 4,000 businesses who used the Eat Out to Help Out scheme to ask them to check their claims.
The scheme saw more than 84,000 food and drink establishments offer a 50% discount on Mondays, Tuesdays and Wednesdays during August, which was refunded to businesses within five days.
HMRC is now undergoing post-payment compliance checks and writing to businesses when its records suggest they may have made an incorrect claim.
The letter sent to businesses says any overpaid payments that are voluntarily repaid will not be penalised, adding: "We are supporting our customers while tackling serious fraud and criminal attacks. We understand mistakes happen, particularly in these challenging times. This means we will not look for innocent errors and small mistakes for compliance action."
However, claimants have 60 days to respond to the letter or HMRC could start a formal compliance check involving statutory interest and penalties. Some may be asked to provide evidence of eligibility and their calculations, and even those who believe their claims were correct need to contact HMRC using the details provided in the letter.