MPs have had their expenses scrutinised still further
Commons Speaker John Bercow has written to all MPs urging them to co-operate fully with an independent audit of their expenses claims since 2004.
He said Sir Thomas Legg's review was part of the process of restoring public confidence in expenses and MPs.
Sir Thomas has written to many MPs to ask them to justify some claims - and some may be asked to repay money.
There have been reports that some MPs plan to defy recommendations they now repay claims approved at the time.
MPs returning to the Commons on Monday after the summer recess have been receiving letters from Sir Thomas, a former civil servant, asking them for more information about some claims.
Sir Thomas, whose final report is expected in December, does not have the power to demand that they repay money.
His report will go to the Commons' Members Estimate Committee which will decide whether to order its recommendations be carried out.
MPs are allowed to claim expenses for running a second home but there was much uproar in May when receipts and details of what they had been claiming for were leaked to a newspaper.
Among them were claims for expensive TVs and furniture, MPs who claimed for more than one property by "flipping" the designated second home and others who over-claimed for mortgages or services.
Many MPs have announced they will be standing down, some have already repaid claims in response to constituents' anger.
Party leaders pledged to change the system and an independent review is due to make its recommendations this month.
The PM also asked an independent auditor to go over past claims again, to ensure money had been paid out properly.
But Sir Stuart Bell, who sits on the estimate committee, told the BBC on Sunday that Sir Thomas had been asked to carry out a review "in accordance with the rules at the time and the standards that applied at the time over the past five years".
"I think many MPs, if they read the newspapers, may feel [Sir Thomas] is not staying within that remit, he's not respecting the decisions that were made by the Fees Office in accordance with the rules at the time."
The Speaker, who chairs the estimates committee, has written to all MPs to urge them to co-operate fully.
He also says Sir Thomas was asked to examine payments "against the rules and standards in force at the time".
Sir Thomas was asked to scrutinise all MPs' claims after details of what they had been claiming under their second homes allowance and others were leaked and published by the Daily Telegraph in May.
The BBC understands he has set retrospective limits for some items and annual limits on what he believes they should have claimed - including £1,000 a year for gardening and £2,000 for cleaning.
Speaking on GMTV on Monday, the prime minister said he would repay money if asked to do so and recommended other MPs do the same.
"We've got to consign the old discredited system to the dustbin of history, this is part of the process of doing so," Gordon Brown said.
"Sir Thomas Legg will make recommendations, people have the chance to look at what he says.
"And then my advice to people is.... [if] he says you've got to repay, let's get it done, let's get it sorted out and let's get it back to a system that people have confidence in."
Shadow Commons leader Sir George Young told the BBC: "The letter we get today is the beginning of a process and not the end and, if you look at his terms of reference, it says that members will have a fair opportunity to make representations."
He added: "We need to reply to the letters, give him the information he wants, make any representations and take the agenda on. We all have an interest in bringing this to a conclusion."
Conservative leader David Cameron added: "Every MP has got to take part in this. This is a very important part of cleaning up our politics, sorting out the mess of the House of Commons, so everyone should take part.
"Everyone should respond to these letters, respond to the inquiries that are being made - and, of course at the end of this process, then everyone will have to comply with what the authorities are asking."
Business Secretary Lord Mandelson said "on one level" he could understand the reaction as MPs had "honestly and fairly" made claims within the rules as they existed at the time.
"For somebody who they thought was being appointed as an auditor but is in a sense rewriting the rules... many MPs will feel today [that it is] desperately unfair," he said.
"But I think they also have to understand where public opinion is on this and, in order to bring closure to this, I think MPs will need to bite on this particular bullet, however painful."
Expenses approved by the Fees Office have been questioned since details of the claims broke and the old system is widely agreed to have been discredited.
All MPs had to sign off their expenses confirming that they "incurred these costs wholly, exclusively and necessarily to enable me to stay overnight away from my only or main home for the purpose of performing my duties" as an MP.
Interim changes have already been made to the system ahead of a wide-ranging review of MPs' expenses by the independent committee on standards in public life, which is due to report back this month.
Many MPs have already repaid some claims following public anger when details were leaked over the summer, when many MPs were accused of extravagance, over-claiming and avoiding tax on home sales.