Gordon Brown may have to pay back some of his expenses
Prime Minister Gordon Brown may have to pay back some of his expense claims, the BBC understands, as all MPs await a review of claims made since 2004.
The BBC's political correspondent Iain Watson said it is "highly likely" that Mr Brown will have to pay back cash.
Whitehall sources have indicated "the majority" of MPs will either have to justify their claims or pay money back.
However one Labour MP, Sir Stuart Bell, expressed concerns about the process and whether MPs will be treated fairly.
Sir Stuart said MPs must be judged by the rules that applied at the time, despite the fact that they are now seen as widely discredited.
After details of claims since 2004 were published earlier this year, MPs - including all three party leaders - have agreed to repay claims totalling hundreds of thousands of pounds.
Mr Brown is among possibly hundreds of MPs due to receive letters this week from the auditor appointed to investigate expense claims over the past five years.
Former senior civil servant Sir Thomas Legg is understood to be writing to the MPs asking them to either give more details on their expense claims or to pay the money back into the public purse.
IAIN WATSON, POLITICAL CORRESPONDENT
The Prime Minister is likely to become a victim of his own clean-up campaign.
Back in May, facing difficult European elections - and a welter of expense allegations in the Telegraph, it was Gordon Brown himself who called for all claims on MPs' second homes to be independently audited.
Now that they have been, it's highly likely he too may have to stump up some cash. Indeed he's already paid back £153 for wrongly claiming twice for a plumbing bill. Any further repayments may be modest but the political damage is likely to be much greater.
Although there is no suggestion of impropriety on his part -and even though his opposite number David Cameron has already paid back nearly £1,000 in expenses himself - polling suggests that voters place more blame on the government than the opposition.
Last May it emerged that Mr Brown had paid his brother Andrew £6,577 for arranging cleaning services for his Westminster flat for 26 months.
At the time, Downing Street said the brothers had shared a cleaner who worked in both their flats. Andrew Brown had paid her wages and had then been reimbursed by the prime minister, who then claimed cleaning expenses.
It was also revealed the prime minister had claimed twice for the same plumbing work within six months of each other. The House of Commons Fees Office said this had been an "inadvertent mistake".
It apologised for having not spotted it and Mr Brown is understood to have repaid the sum involved - believed to be £150.
It was also reported that Mr Brown may have billed taxpayers for his Sky Sports subscription.
On Saturday Mr Brown told a newspaper that "the worst offenders" among MPs who claimed bogus expenses should be prosecuted.
After the claims were made public in the Daily Telegraph earlier this year, many MPs were accused of extravagance, over-claiming and avoiding tax on home sales.
Several were found to have repeatedly "switched" their designated second homes, meaning they were able to refurbish both their homes at public expense.
The BBC understands that Sir Thomas - who was appointed by Downing Street - has been looking at whether MPs were using money to improve their properties, rather than just maintain them as the rules allow.
He is also thought to have examined instances where MPs used public money to pay off the capital on their mortgages, rather than mortgage interest.
WITHIN THE RULES
£24,000-a-year Additional Costs Allowance, which covers the running of MPs' second homes
£22,193-a-year Incidental Expenses Provision, which pays for running an office
£10,400-a-year Communications Allowance, which funds websites, newsletters, stationery and postage
It is understood Sir Thomas will allow MPs three weeks to respond to questions over their claims but once that process is complete, his full report will be made public, probably in December.
Newspapers have claimed more than 300 MPs could be facing further queries about their expense claims although these figures could not be verified independently.
Sir Thomas has previously said he would judge individual claims by the standards that applied at the time and take into account the fact they were approved by the Commons authorities.
"In so far as Sir Thomas stays within the remit, then MPs would be very happy to comply," Sir Stuart Bell, who sits on Parliament's Members Estimate Committee, told the BBC.
"But I think many MPs, if they read the newspapers, may feel he is not staying within the remit, he is not respecting the decisions made by the Fees Office in accordance with the rules at the time."
Work and Pensions Secretary Yvette Cooper said that while future claims would be examined externally, an independent audit of past claims was a "good thing".
"We have changed the law, we have made a lot of changes for the future but we all know that this whole thing has been a real mess," she told Sky News.
Shadow home secretary Chris Grayling said that every MP must take responsibility for their claims if Parliament was to "purge our politics" of public mistrust.
The MP for Epsom and Ewell is one of a number of Tories to agree to forego the allowance for a second home following the public outcry.
"I believe the best way for us to deal with this is to go through a full audit of what has been done and if we have done things wrong and if we have claimed money inappropriately, then clearly we should repay it," he told the Politics Show.