Further inquiries into MPs' expenses are being carried out
MPs are facing a further challenge to their expense claims when Parliament resumes after the summer recess.
Many will receive letters from an official investigator who has been looking at MPs' claims dating from the past five years, the BBC has learnt.
The MPs will be asked to justify their claims, and may be asked to provide further evidence to back them up.
In an interview in the Daily Telegraph, Gordon Brown has said he thinks "the worst offenders" should be prosecuted.
International Development Secretary Douglas Alexander told the BBC that MPs had a duty to respond to the letters.
After the publication of hundreds of claims earlier this year, several MPs announced they would stand down.
It followed widespread public disgust after their expense claims were published in the Telegraph.
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.
Following the Telegraph's revelations, former senior civil servant Sir Thomas Legg was commissioned by Downing Street to examine the MPs' claims.
Prime Minister Gordon Brown originally said four years of claims made on second homes would be re-examined to see if they complied with the rules in place at the time.
However that has now been extended to a five year period.
The BBC understands that Sir Thomas is interested in 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.
The Times newspaper said on Saturday that up to 100 MPs could be facing further queries about their expense claims.
So far, more than a dozen MPs have said they planned to stand down after the controversy over expenses first began.
Many others said they would repay money, and so far more than £300,000 has been handed back to the exchequer.
In his interview with the Daily Telegraph, Mr Brown said he believed: "the worst offenders should face prosecution."
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
He added: "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.
"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."
BBC political correspondent Iain Watson says Sir Thomas will allow MPs to make "fair representations" if their claims are challenged - but once that process is complete, his full report will be made public, probably in December.
Mr Alexander said: "This is the next stage in a process. There's been an independent audit of everybody's claims...
"If there are questions to be answered then I hope every member of Parliament answers them."