Thousands of people are receiving letters from HMRC questioning their claims for coronavirus support grants.
The questions surround whether these people had been trading at the required times.
Some 2.7 million people claimed grants as part of government support.
The first was worth 80% of profits up to a cap of £2,500 per month, and the second covering 70% of profits, up to a cap of £2,190 per month.
The letters are sent to people who had told HMRC that, at some point, they had stopped trading, and so would only be eligible if they had restarted.
“If they are trading and have claimed correctly, we may ask them for extra documentation to show this,” the tax authority said.
“If they had stopped trading but still made a SEISS claim, they must reply to the message by 20 November to avoid a penalty.”