Efforts to force MPs to pay back their expenses were seriously undermined yesterday when Sir Thomas Legg, the auditor who sparked their fury, admitted having made a mistake.

He issued an unreserved apology to Kenneth Clarke and reduced the demand sent to the Shadow Business Secretary by more than £3,000. Sir Thomas’s aides also admitted that there were errors of detail in demands sent to other MPs.

The episode gave some MPs hope that their appeals to Sir Thomas might succeed and reinforced others in their view that his trawl was flawed.

Gordon Brown and David Cameron have urged their MPs to pay up, but an increasing number plan to fight Sir Thomas’s retrospective limits for claims on gardening and cleaning. The MPs argue that, since the claims were approved at the time, it is unjust to change retrospectively the limit of what could be claimed.

Many are not expected to stand at the election and some claim that they would face financial hardship, or even ruin, if forced to return large sums.

One Labour MP expressed delight that Mr Clarke’s appeal had succeeded. “It is more proof that the Legg inquiry was all over the place. It is discredited,” the MP said. A spokesman for Mr Clarke said that he would abide by the final conclusions of the review of claims made since 2004.

Last week Sir Thomas suggested that Mr Clarke return £4,733 of legitimately claimed expenses. But yesterday he said that the figure for the Tory MP for Rushcliffe should have been £1,345, and last night his spokesman said: “In a small number of cases where there have been errors we have apologised.”

John Mann, the Labour MP for Bassetlaw, said that he also received a correction letter from Sir Thomas after querying a request for payment of £110, and now owes nothing.

When Sir Thomas’s letters went out last week, several MPs said that mistakes had been made and some alleged that they were being asked to repay money they had never claimed.

Sir Thomas has written to about 600 MPs after reviewing claims following the expenses scandal. Many have been asked to repay sums approved at the time.

Although there were no caps on gardening and cleaning bills when the claims were made to the Commons Fees Office, he said that there should have been, to prevent “disproportionate” payments.

The Conservative leadership published what its front bench had been asked to repay and Mr Clarke had been asked for the most. All MPs were given three weeks to respond to the requests and Mr Clarke indicated that he would “make representations”.

Sir Thomas said in a letter to Mr Clarke: “The error arose because some of your claim forms are duplicated in your file and the amount claimed for cleaning was included in calculations twice.

“This should not have happened and I want to lose no time in offering an unreserved apology.”

No 10 said that Mr Brown would not quibble over the £12,000 that he returned to Sir Thomas to cover his excessive cleaning expenses.

The Prime Minister’s spokesman said it was “not a surprise that there are these inconsistencies” given the volume of material that was reviewed. It remained Mr Brown’s position that MPs should take up any queries over the findings with Sir Thomas but then accept his final recommendation, he said.

Yesterday Downing Street denied that the Prime Minister had proposed a pay rise for MPs. However, members in all parties argue that with Sir Thomas arbitrarily cutting their approved claims and Sir Christopher Kelly likely to pare back allowances further, it is time for the Senior Salaries Review Body to recommend an increase.

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