A rebellion among MPs gained momentum in Westminster today as politicians from all three main parties ignored their leaders and publicly questioned the authority of an independent auditor who is demanding the return of overpaid expenses.

Senior MPs cast doubt on the legal basis for retrospective claim limits set by Sir Thomas Legg and are suggesting that the audit of their allowances has been rushed and inaccurate, despite pressure from whips to enforce Sir Thomas’s requests immediately.

While backbenchers queried the process, details of the letters sent by Sir Thomas to Cabinet and Shadow Cabinet members emerged.

The Conservative Party indicated that they would publish details of their entire frontbench this evening but the contents of letters to Liam Fox, Andrew Mitchell and David Mundell remain a mystery. A Tory party aide said discussions on those three sets of expenses were ongoing, while the rest had agreed to repay around £14,000 between them.

Ann Widdecombe, a former Conservative minister, said there was “a big question of the legality” over the decision, which is expected to result in five or even six-figure repayment demands for some MPs.

Her complaint was followed by Labour MP Martin Salter, who said the process was “monstrously unfair”, and Norman Baker, a Liberal Democrat MP, who said he was concerned by fundamental errors which may have been made in haste by Sir Thomas.

Labour's Bill Etherington, the Sunderland North MP, became the first refusnik of the affair when he warned he would not pay back any money claimed under the rules as they stood at the time. "They can take the matter to court," he told the Sunderland Echo.

When the review was set up in the wake of the parliamentary expenses scandal this spring, Sir Thomas, a retired civil servant, was thought of as a safe pair of hands who would ensure that the issue faded away. However, he has proved unexpectedly robust in his judgments on MPs' claims for expenses for second homes.

Some backbench MPs have failed to contain their anger at the move. Mr Baker, MP for Lewes, told The Times that he had been asked for additional documentation over mortgage repayments despite having no mortgage and making no claims for one.

“The reason he’s got no information is I don’t have a mortgage,” he said, adding that he was worried at “such a fundamental error”.

“It strikes me this is a bit of a rush job and there are some inaccuracies. If you get that wrong, maybe it’s not £12k Gordon Brown owes, maybe it's £11k or £13k? You wonder whether you have trust in the system.”

The Prime Minister last night paid back £12,415 for cleaning, gardening and decorating at the properties he has designated at various times as his second home. David Cameron and Nick Clegg joined him in responding immediately to requests from Sir Thomas after receiving letters about their own expenses.

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