MPs face having virtually all of their expenses broken down for publication after a ruling by Information Commissioner Richard Thomas.
He ordered the House of Commons to release the details of several MPs' claims in a move that could open the floodgates to similar demands relating to all parliamentarians.
Allowance claims for the MPs' staff, IT, stationery and the cost of living away from home should be published in detail, Mr Thomas said. Currently only the totals under each heading are released.
While his ruling applied to only six - including Prime Minister Gordon Brown and predecessor Tony Blair - a source said it was indicative of how the Commissioner would treat requests pertaining to all MPs.
The Commons, which originally rejected the request under the Freedom of Information Act in 2005, could now appeal against the ruling to the Information Tribunal. A previous appeal concerning MPs' travel expenses failed last year, however.
A similar ruling on the additional costs allowance - generally covering the costs of second homes for MPs with constituencies outside London - is currently being appealed.
MPs have faced sustained criticism for the size of their expenses claims, which averaged £135,800 each last year and - apart from travel - are not explained in any detail to the public.
In a statement, Mr Thomas' office said: "In the Commissioner's view the legitimate public interest in disclosing the information outweighs the prejudice to the rights, freedom and legitimate interests of MPs."
His decision was made on the grounds that such expenses arose from the role of MPs as public representatives and that they were reimbursed by the taxpayer.
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The decision follows a request for the detailed spending between April 2003 and March 2004 of Mr Blair, Mr Brown, former deputy prime minister John Prescott, ex-Tory leader Michael Howard, former Liberal Democrat leader Charles Kennedy and ex-Tory MP Jonathan Sayeed.
