House of Commons officials have today called in the police to hunt down the mole who leaked details of MPs expenses.
The parliamentary officials spent the morning in talks with Scotland Yard, and made the decision this afternoon.
In a statement, officials said: "The House authorities have received advice that there are reasonable grounds to believe a criminal offence may have been committed in relation to the way in which information relating to Members' allowances has been handled.
"A report has been made to the Metropolitan police, asking them to consider the matter."
Even before the details of MPs' expenses were splashed on to the Telegraph's front page and nine inside pages, a hunt was under way for the person who had earlier this year leaked information about claims made by MPs.
The House of Commons commission, responsible for publishing a full list of MPs' expenses planned for July, heard that a computer disk containing the details was allegedly stolen and being offered to newspapers.
The Labour MP Sir Stuart Bell, a member of the commission, said the information was being offered to national newspapers for up to £300,000 in March and that a hunt had been launched to find the mole.
Bell said: "All of the receipts of 650-odd MPs, redacted [edited] and unredacted, 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."
The commission today confirmed an investigation had been launched after details of the home secretary's claims were leaked. Infamously they included a claim by her husband for adult films.
The commission said: "An investigation is being done by the house authorities. If there is a prima facie case of criminal activity we would ask the police to become involved."
The spokesperson refused to reveal who would be carrying out the investigation but said it was limited at this stage to the earlier leaks.
Police could have decided to launch an investigation without receiving a complaint, but that was thought unlikely given the force's unhappy experience of investigating Westminster in recent years.
The police investigation into a mole passing information to the Tory frontbencher Damian Green was roundly criticised and MPs were also angered by the protracted cash-for-peerages investigation.
The former Liberal Democrat leader Menzies Campbell earlier told the Guardian he was against the idea of the police being called in. "I'm not sure a leak inquiry would serve any purpose," he said. "Indignation from MPs I don't think will carry much weight on this topic."
Suspicion over who was responsible for the leak today fell on a City businessman reported to have offered the details of MPs' expenses to national newspapers in March for up to £300,000. It was also suggested that a middleman for someone with access to the information was hawking the details of cabinet ministers' expenses for £10,000 each.
The opportunity for an unauthorised person to get their hands on the details arose because of a decision to publish a full list of expenses this summer, following a freedom of information request.
As part of the process, receipts were digitised by staff at the government's Stationery Office. A businessman who offered one newspaper the details said the details were accessible because a copy had been accidentally made without any of the officials realising.
MPs have access to their own records via a supposedly secure internet site or can request a hard copy. They had until 21 May to report inaccuracies or ask for sensitive information to be edited out.
The Right Honourable Michael Martin MP, the Speaker of the British House of Commons, faces a vote of no confidence this week. This motion will be launched by Tory Douglas Carswell MP. Martin addressed the House of Commons at 15:30 BST (14:30 UTC).

The Right Honourable Michael Martin MP in 2008Speaker Martin has faced much controversy over his handling of a recent scandal involving the expenses of Members of Parliament (MPs). Martin defended the House of Commons' authorities that requested a police investigation with regard to this leak, causing anger amongst many MPs.

In his statement, the Speaker apologised to the public saying that "we have let you down". He also personally apologised: "I am profoundly sorry". He acknowledged that "each and every member, including myself, must work hard to regain your trust".

Sir Christopher Kelly will release a report in Autumn about expenses, and the Speaker said that the House should take steps until then and MPs should not file further expense claims. Some MPs urged an interim report to be published before this. The Speaker urged leaders of all Parliamentary parties, including the Prime Minister, to meet with the Speaker and House of Commons Commission within 48 hours to discuss expenses.

After the speech, various MPs asked the Speaker questions. Carswell asked "when will members be able to choose a new leader?" and David Winnick MP asked for information about when the Speaker will retire. Bob Spink, the Independent MP for Castle Point, said that his constituents did not want the Speaker to be a "scapegoat". Labour MP for Middlesbrough, Sir Stuart Bell, said "the majority of the House will support this statement you made today" and noted "never in history has there been such an attack on Speaker".

In the UK, attacking the Speaker defies a long-standing tradition; the last Speaker to be forced from office was Sir John Trevor in 1695 for accepting a bribe.

Even before the speech, MPs from all major parties have supported this motion; Nick Clegg, the leader of the Liberal Democrats, has urged the speaker to resign. On his blog, Carswell claimed that he 'continues to pick up support for the motion'.

Martin's friend, the Labour Lord Foulkes, has suggested Martin is a victim of snobbery, as the Speaker is "someone from a working class background in Glasgow".

In the past, Martin has been involved in several past scandals, including that he made excessive claims for the use of taxis, and for over £1,600,000 spent on the Speaker's residence.