Sir Stuart Bell: "We will get to the bottom of it"
Details of MPs' expenses receipts are being offered for sale for up to £300,000, it has been claimed.
Labour MP Sir Stuart Bell said the Speaker's Commons Estimates Committee he sits on was investigating the case.
In the wake of a string of allegations about expenses, Prime Minister Gordon Brown has suggested scrapping MPs' controversial second-home payment.
As questions continue about MPs' claims the Committee on Standards in Public Life said it would speed up its review.
It had planned to start its investigation later this year but the leaders of the three biggest political parties had all called for it to be brought forward so it could recommend changes before the next election.
The suggestion that details of MPs' expenses claims were being offered for sale comes after weekend revelations that Home Secretary Jacqui Smith "mistakenly" claimed for adult movies watched by her husband.
Sir Stuart told BBC Radio 4's Today on Tuesday: "All of the receipts of 650-odd MPs, redacted and un-redacted, are for sale at a price of £300,000, so I am told.
"The price is going up because of the interest in the subject."
It may be a theft, but we will get to the bottom of it. In the public interest
Asked who was trying to sell the information, Sir Stuart replied: "Well we have a pretty good idea of not the person, but the source, and that is a subject of a House of Commons investigation."
"It's probably a breach of the Official Secrets Act," Sir Stuart said. "It may be a theft, but we will get to the bottom of it. In the public interest, by the way."
He told BBC Radio 4's Today programme: "We don't believe that the newspapers will buy all of the 655 for £300,000, but they may wish to take say 10 senior MPs in the limelight and then there'll be a drip-drip of their expenses.
"All of these expenses will be published, all of the receipts will be published, probably in July of this year, so there'll be 1.2 million receipts on the table and they can all have a field day."
Sir Stuart added that all three of the main political parties should commit to reform of MPs' expenses in their manifestos for the next general election.
Newspapers have reported a string of allegations about MPs' expenses in recent weeks, fuelling speculation that information might be being leaked by officials within Parliament.
The allegations have prompted calls for a reform of the additional costs allowance - known as second-home 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 showed the total expenses claimed by MPs went up 6% in 2007-8, to just over £93m.
In a letter to the independent Committee on Standards in Public Life, Mr Brown urged it to consider replacing the existing payment with a "simpler overnight allowance" that would be independently determined.
He also suggested that 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.
In his letter to the committee's chairman Sir Christopher Kelly, 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."
He suggested the committee considered scrapping the controversial second-home allowance in favour of a simpler system.
London MPs should be prevented from claiming for second homes altogether, he indicated, and others moved to "a simpler, overnight allowance that is independently determined".
The prime minister's intervention came after allegations about allowances claimed by Employment Minister Tony McNulty and the home secretary.
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 circulating to members for them to check and make representations on which parts should be deleted for security and other reasons.
They are expected to prove damaging to many MPs when they are published before the summer recess.
It emerged that Mr McNulty claimed the allowance towards his parents' home in Harrow which he used in addition to a flat in central London.
Ms Smith was criticised for claiming more than £116,000 in allowances for her family home in the West Midlands after nominating her sister's property in London - where she stayed several nights a week - as her main residence.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.