Scores of MPs have been angered by letters from independent auditor Sir Thomas Legg, calling on them to pay back expenses previously approved by the Commons Fees Office.
Senior parliamentarians have told back-benchers there could be a vote in the Commons to scupper the demands of the audit if Sir Thomas, a former civil servant, does not back down.
Labour politicians, in particular, are outraged that the Conservatives have, as yet, escaped most of the public scrutiny.
But the Prime Minister – who has so far come in for the highest bill of any MP, having to repay 12,400 for an over-claim on cleaning – urged his party to settle its bills.
Mr Brown has warned he could withdraw the whip from any Labour MP who refused to accept Sir Thomas's findings.
"If people are not prepared to co-operate, we will have to consider that action," he said.
Tory leader David Cameron also sent a warning to MPs on his benches who were tempted to argue against the demands.
"If people are asked to pay back money and if the authorities determine that money should be paid back and they don't, in my view they can't stand as Conservative MPs," he said.
Senior parliamentarians are believed to be considering forcing a vote on Sir Thomas's audit, which could be vetoed if there were enough "inaccuracies".
One Scottish MP said: "Some MPs have good reason to feel aggrieved. It also seems a bit unfair that Labour MPs who pay their cleaners a fair wage are more likely to get caught out, while Tory millionaires are getting away with charging taxpayers for their mammoth mortgages on their country estates."
Former Tory minister Ann Widdecombe, who has some of the most modest claims of any MP, warned there was "a big question of the legality" of the decision, which is expected to result in five- or even six-figure repayment demands for some MPs.
Miss Widdecombe said: "If any other employer did this, he would be up before a tribunal. I have spoken informally to a number of practising lawyers, and they say that it is contrary to the rules of natural justice."
Meanwhile, the SNP published all of its MPs' expenses late on Monday night.
First Minister Alex Salmond has had to pay back 710 in moving costs from Westminster to Aberdeenshire after the Scottish elections.
He has also been asked to supply more details over a 2,610 hotel bill, claimed while his rented London flat lay empty.
Mr Salmond yesterday said he did not want to "quibble" about his bills. "When you ask for a referee, you have to abide by the referee's decision," he said.
But many MPs are incensed at the way Sir Thomas is applying the rules retrospectively.
He is said to have resorted to making up his own rules, after finding there were no clear limits in place for cleaning and gardening costs.
He decided to set a 2,000 maximum for cleaning, while gardening expenses were limited to 1,000 a year.
"Some limits must be regarded as having been in place to prevent disproportionate and unnecessary expenditure from the public purse," Sir Thomas wrote in a letter to all MPs.
Politicians have been given three weeks to respond to his findings, and he is expected to deliver his final report to the members estimates committee (MEC), which oversees expenses for the Commons authorities, on 9 November.
Labour back-bencher Martin Salter – who is unaffected by the process as he does not have a second home – warned some MPs were so angry they could mount a legal challenge.
"Any attempt to apply a retrospective value judgment is undoubtedly going to be subject to challenge," he said.
"Far from drawing a line under this appalling situation, which has dragged politics into disrepute, it is going to make the situation many times worse."
But Sir Stuart Bell, a member of the MEC, dismissed the idea of legal action as "balderdash", saying there was "absolutely no prospect" of Sir Thomas's report being thrown out.
The Tories have suffered some high-profile expense demands, with 11 shadow cabinet members being asked for repayments.
Shadow business secretary Ken Clarke has been asked to return more than 4,000 he claimed for gardening and cleaning, while shadow health secretary Andrew Lansley has been asked for a further 1,782.22, taking his total repayment bill to 4,423.92.
Senior Cabinet members have also had to pay back cash.
Justice Secretary Jack Straw has had to pay back 600 for a double bill. Treasury Chief Secretary Liam Byrne is returning 1,860.54 for a phone bill submitted on the wrong claim form, and for letting-agency charges. Business Secretary Lord Mandelson is to pay back 800 he claimed for tree surgery in 2004, when he was MP for Hartlepool.
Challenges, information – and even repayments
HALF of all Scots MPs responded to queries about their expense letter from Sir Thomas Legg by last night. The outcome so far has been:
&149 Danny Alexander, Lib Dem, Inverness, Nairn, Badenoch and Strathspey: Asked for details about bills; will repay 125 claimed for mortgage advice.
Gordon Banks, Lab, Ochil and South Perthshire: Is providing mortgage papers.
Anne Begg, Labour, Aberdeen South: Cleared.
Des Browne, Labour, Kilmarnock and Loudoun: Query on furniture and mortgage interest charges.
Gordon Brown, Lab, Kirkcaldy and Cowdenbeath: Repaying 12,400 cleaning costs.
&149 Malcolm Bruce, Lib Dem, Gordon: Responding to queries over 3,150 for blinds; challenging demand for repayment of 300.
Alistair Carmichael, Lib Dem, Orkney and Shetland: Responding to request for mortgage statements.
Tom Clarke, Labour Coatbridge, Chryston and Bellshill: Cleared.
&149 Alistair Darling, Lab, Edinburgh South West: Repaid 554 for chest of drawers; is providing mortgage details.
Jim Devine, Lab, Livingston: Cleared.
Frank Doran, Lab, Aberdeen North: Will answer query about an amount of around 200.
Brian Donohoe, Lab, Ayrshire Central: Giving clarification on mortgage payment.
Tom Harris, Lab, Glasgow South: Cleared.
Stewart Hosie, SNP, Dundee East: Repaying 379 hotel costs.
Charles Kennedy, Lib Dem, Ross, Skye and Lochaber: Queries asked on expenses.
&149 Angus MacNeil, SNP, Western Isles: Repaying partial council tax bill of 133; challenging repayment of hotel bill.
David Mundell, Conservative, Dumfriesshire, Clydesdale and Tweedale: Asked for paperwork on rental agreement.
Jim McGovern, Lab, Dundee West: No letter received yet.
Anne McGuire, Lab, Stirling: Responding to query about rental records.
Rosemary McKenna, Lab, Cumbernauld, Kilsyth and Kirkintilloch East: Not commenting before deadline.
Anne Moffat, Lab, East Lothian: Asked for further information on expenses.
Alan Reid, Lib Dem, Argyll and Bute: Cleared
Angus Robertson, SNP, Moray: Repaying 1,217 for sofabed and DVD recorder and providing statements.
&149 Alex Salmond, SNP, Banff and Buchan: Repaying 710.88 for removal costs; replying to queries about hotel costs of 2,610.
Mohamad Sarwar, Lab, Glasgow Central: Clarification on a 235 duplicate bill and mortgage information.
Sir Robert Smith, Lib Dem, West Aberdeenshire and Kincardine: Asked for more information.
Gavin Strang, Lab, Edinburgh East: No letter received yet.
John Thurso, Lib Dem, Caithness, Sutherland and Easter Ross: Responding to council tax query.
Mike Weir, SNP, Angus: Supplying copy of rental agreement for his London flat.
Pete Wishart, SNP, Perth and North Perthshire: Paying 1,632 for a duplicate claim for flat rental and a utility bill.
Jo Swinson, Lib Dem, Dunbartonshire East: Asked for details on second home payments.
Why has the expenses issue come up again?
After details of MPs' expenses claims were leaked to the Daily Telegraph in May, Sir Thomas Legg was asked to conduct an audit of all MPs' claims over the past five years. He wrote to them with his provisional findings on Monday.
Is this finally the end of the expenses row?
Not necessarily. Sir Thomas is requiring some MPs to repay expenses that were permissible at the time, so not all of them may be willing to return the money. Some also feel he has set arbitrary limits on how much they should have spent for certain things, such as 2,000 a year for cleaning and 1,000 for gardening.
Could any MPs face legal action over their claims?
Police are investigating alleged misuse of expenses by a small number of MPs and peers. MPs are also threatening to legally challenge Sir Thomas's findings.