The expenses scandal will return to haunt the Commons next week when MPs are set to be sent letters about their claims over the past five years.
Auditors are expected to ask up to 100 members to justify instances where they have received public money, or to repay it.
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.
In an interview with the Daily Telegraph, 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."
Numerous MPs from across parties have already announced they are stepping down after being caught up in the expenses row, and Scotland Yard is considering whether activities by some may have been criminal.
