The audit of MPs' expenses by Sir Thomas Legg, little mentioned over the past few months, turned out to be a massive parliamentary time-bomb just waiting to explode at Westminster.
Hundreds of MPs have received letters from Sir Thomas calling on them to repay some of the expenses paid over the past five years.
Gordon Brown is handing back more than £12,000. The Lib Dem leader, Nick Clegg, is re-paying about £900. The Conservative leader, David Cameron, has been asked for more information about his mortgage payments. Even the Commons Speaker, John Bercow, has been asked to repay nearly £1,000.
The Legg letters have caused fresh fury at Westminster because, say MPs, Sir Thomas has exceeded his brief - and made up his own rules after the event.
Sir Thomas argues that the old limits on claims for things like gardening and house cleaning at MPs' second homes were so vague as to be almost meaningless. So he has worked out what he thinks the limits should have been - and applied them retrospectively.
He has ruled, for example, that cleaning bills should be limited to £2,000 a year and gardening to £1,000 a year.
MPs insist, though, that their claims were approved at the time and they say that to impose arbitrary new limits now is contrary to the rules of natural justice.
And they want to know why these limits cover gardening and cleaning but not mortgage repayments.
Gordon Brown has urged Labour MPs to pay up. And David Cameron said that any Tory MP who refuses to make repayments would be barred from standing at the next election.
But a number of MPs seem to be spoiling for a fight.
Some have been saying they will refuse to pay up.
Others are threatening to tell Sir Thomas: 'We'll see you in court.'
And following the uproar, MPs are now being given the chance to lodge an appeal against Legg.
The Commons Estimate Committee, which considers MPs' pay and allowances, has asked a former Court of Appeal judge, Sir Paul Kennedy, to consider complaints from MPs.
The appeals process is due to be completed in early 2010 - when its findings will be published alongside the final report from Sir Thomas Legg.
