More than 300 UK MPs could be asked either to repay money or provide further information to justify their expenses claims, it has been reported.
MPs are set to receive letters next week about their claims over the past five years.
Auditors are expected to ask up to 325 members to justify instances where they have received public money, or to repay it, the Sunday Telegraph reported.
The letters are reportedly being sent out by former civil servant Sir Thomas Legg, who has been leading a review of all claims since 2004. Sir Thomas is believed to be examining cases where MPs have used parliamentary expenses to improve their second homes and make a profit, rather than just maintain them.
He is also said to have uncovered more examples where taxpayers' money has been used to pay off the capital element of mortgages, instead of just interest on the borrowing, as is allowed under the rules.
The letters will be sent out privately, and Sir Thomas is not expected to deliver his final report until December.
They will also receive an email containing a detailed analysis of their use of the Additional Costs Allowance, which is intended to help meet the costs of running a second home, the newspaper reported.
MPs will be told they have three weeks to challenge Sir Thomas's findings and can appeal to the Commons' standards and privileges committee if they do not agree with his conclusions.
According to the BBC, the Prime Minister could be among those asked to pay back cash. In an interview, Gordon Brown said he believed the "worst offenders" in the scandal should be prosecuted.
"It's right to distinguish between what you might call corruption in some cases, which is for the courts to decide, and honest mistakes or misunderstandings about rules that are very unclear," he said.
"Some cases will end up in the courts, where someone's done something very wrong we've got a duty to deal with them most severely."