There have been widespread calls for reform of the system
The Committee on Standards in Public Life has announced it plans to bring forward its inquiry into MPs' expenses.
Chairman Sir Christopher Kelly said it was "now obvious" work had to go ahead "as soon as possible".
It comes as Speaker Michael Martin said he was "deeply disappointed" expenses details were leaked to the media.
He told MPs 1.3 million receipts had been handed to a private contractor "in good faith" and an inquiry had been launched to find out what had happened.
The Commons probe will "establish the facts relating to the release of personal documentation and to determine whether an offence has been committed", Mr Martin told MPs.
It comes amid reports that all of the receipts are being offered for sale to the media for as much as £300,000.
Labour MP Sir Stuart Bell, who sits on the Speaker's Commons Estimates Committee, told the BBC it was investigating the reports.
And the Information Commissioner Richard Thomas said he "deplored" the unauthorised leak or sale of information.
It is understood the receipts were given to the contractor, which had been security vetted, to be processed prior to their release later in the year, to comply with a Freedom of Information request.
The standards committee's decision to bring forward its inquiry follows newspaper revelations about the use of second-home allowances by ministers.
Home Secretary Jacqui Smith was criticised for claiming more than £116,000 for her family home in the West Midlands after nominating her sister's property in London - which she shared - as her main residence.
It has emerged that she also mistakenly claimed expenses for adult films watched by her husband at the house in her constituency of Redditch, Worcestershire. The money has been repaid.
Earlier this month it emerged that employment minister Tony McNulty had claimed the second-home allowance towards his parents' house in Harrow, north-west London, which he used in addition to a flat in central London.
Sir Christopher told the BBC: "It may be the case that, in the past, some Members of Parliament regarded expenses as part of their salary in the belief that they weren't paid enough...
"I've no doubt the recommendations we produce will be challenging."
He added: "We intend to do a fundamental, wide-ranging and independent review."
He said there were several different options to consider but that people would "find it hard to understand that any group of people could take responsibility for fixing their own allowances".
In a statement, Sir Christopher said: "It's now obvious that this piece of work needs to start as soon as possible. We're going to defer work on the current inquiry to begin work immediately."
He also said: "The situation has changed quite dramatically over the last few months and I am pleased that there is now such widespread political consensus on the need for reform."
Sir Christopher added: "This is not something which can be done with a quick fix; we want to do a thorough piece of work. Our firm intention is to publish our report towards the end of the year."
However, several MPs say the situation needs resolving sooner.
"There is a reform agenda waiting to be implemented," said Labour's Tony Wright, chairman of the public administration committee, urging the committee to report this summer.
"Exactly what we want is a quick fix," he told the BBC.
"The system is broken. It needs fixing urgently before we become an absolute laughing stock."
Lib Dem MP Norman Lamb said Parliament would "destroy itself" in the public's eyes unless changes to the system of MP's allowances were agreed soon.
"We must show we are capable of reform," he said.
Newspapers have reported a string of allegations about allowances in recent weeks, fuelling speculation that information might be being leaked by officials within Parliament.
These have prompted calls for a reform of the second-home allowance - known as the additional costs allowance - which covers up to £23,083 of the cost of an MP staying away from their main residence to perform their parliamentary duties.
The row intensified when official figures published on Monday showed the total expenses claimed by MPs went up 6% in 2007-8, to just over £93m.
In his letter to Sir Christopher, Mr Brown urged the committee to consider replacing second-home payments with a "simpler overnight allowance" which would be independently determined.
He also suggested all London MPs should receive the same allowance, ending the current distinction which allows those in outer London to make the additional claim for a second home.
Mr Brown said: "I would be grateful if you could look to both start and conclude the review earlier than previously indicated to allow us to make progress on the issue as soon as practical".
WHAT IS THE COMMITTEE ON STANDARDS IN PUBLIC LIFE
The Committee was set up by John Major in 1994, after a series of scandals involving MPs, to advise government on ethical standards. It is funded by the Cabinet Office but independent of government. It has ten members, approved by the prime minister. Sir Christopher Kelly became chairman in 2008
The Commons authorities have been working to prepare around a million receipts filed by MPs for publication, after losing a long-running freedom of information battle to keep them secret.
The documents have now started being circulating to members for them to check and make representations on which parts should be deleted for security and other reasons.
Last July, MPs voted against a proposal by the Members Estimate Committee to replace the second-home allowance with an overnight expenses allowance of £19,600 a year for accommodation.
They also voted to have their spending looked at only by internal, rather than external, auditors.
The Standards Committee, set up in 1994, has 10 members.
Three of these - Labour MP Alun Michael, Conservative MP Oliver Heald and Lib Dem peer Baroness Maddock - were appointed on the recommendation of their party leaders.
Its other members are independent of any political party.