The House of Commons has been ordered to provide a detailed breakdown of MPs' second home expenses claims, after a lengthy Freedom of Information battle.
The current Additional Costs Allowance (ACA) system is "deeply unsatisfactory" the Information Tribunal ruled.
It said the "laxity" of rules on the allowance was "very different" from those in the private sector.
Commons resources boss Andrew Walker had argued publishing more details could intrude on MPs' private lives.
But the tribunal ruled in favour of Freedom of Information campaigner Heather Brooke, the Sunday Telegraph and Sunday Times - and ordered the Commons to release the information on 14 MPs - including Tony Blair, David Cameron and Gordon Brown - within 28 days.
'Lack of clarity'
It is thought the same principle will apply to requests relating to other MPs' claims.
The tribunal heard that MPs claim ACA to cover the costs of staying overnight away from their main home, including rent, hotel bills and mortgage interest payments.
"In our judgment these features, coupled with the very limited nature of the checks, constitute a recipe for confusion, inconsistency and the risk of misuse"
Each MP can claim about £23,000 a year and can submit claims of up to £250 without a receipt and up to £400 a month for food.
The tribunal noted that the guidance available to MPs on what they can claim is "incomplete", that MPs are not trusted to have access to the list of acceptable costs "lest the maximum allowable prices become the going rate" and there are no "additional" checks on what MPs claim.
In its ruling, the tribunal said it was not its job to say how the system should work, but it had to make a judgement on existing controls.
It said: "The laxity of and lack of clarity in the rules for ACA is redolent of a culture very different from that which exists in the commercial sphere or in most other public sector organisations today."
Transparency 'shortfall'
It said that historically MPs were allowed to self-certify, but even if that was acceptable in "modern conditions" it was inadequate as MPs do not have access to a "coherent and comprehensive statement of their entitlements".
It added: "In our judgment these features, coupled with the very limited nature of the checks, constitute a recipe for confusion, inconsistency and the risk of misuse."
It said the current system was "deeply unsatisfactory" and suffered an "acute" shortfall in transparency and accountability.
"All honest and hard-working MPs will welcome this opportunity to prove their openness to the electorate"
Liberal Democrat MP Norman Baker, who won his own FOI battle to get MPs' travel expenses broken down, told the BBC the judgment had been "inevitable".
"It's an allowance that allows MPs, and four fifths of MPs do, to buy a property with public money and to keep the capital gains receipt at the end when the property is sold. I'm afraid MPs have been open to this sort of challenge from Freedom of Information campaigners for some time."
Security details
The tribunal ruled that some details should not be published, including "sensitive personal data" such as MPs' health matters, MPs' bank, loan and credit card statements, individual numbers on itemised phone bills and details of contractors who had regular access to MPs' homes.
Security details will also be kept private, as will addresses of MPs who have a good reason - for example a known stalker, terrorist or "other criminal threat".
Ms Brooke, who joined journalists Jon Ungoed-Thomas and Ben Leapman's three year fight to get the details published, said: "This ruling will wrest control from the old boys' club and put it back where it belongs ¿ with the constituents."
"All honest and hard-working MPs will welcome this opportunity to prove their openness to the electorate."
'Taking advice'
But Labour MP Ken Purchase told the BBC: "I really don't see why I should be pestered on an almost daily basis, asking about this, that and the other - where will it end? Will someone want to know how much I'm paying for dinner tonight? And what purpose will it serve?"
The ruling came on the same day that the Commons Members Estimate Committee said its own review of MPs' expenses will be completed several months earlier than planned - by July.
That was set up following revelations that Tory MP Derek Conway had made payments worth £40,000 to his son for work as a parliamentary researcher while he was a student.
The tribunal said it was not surprised, from what it had seen, that the prime minister wanted a "root-and-branch review".
A spokesman for the Members' Estimates Committee (MEC), which deals with allowances on behalf of the Commons, said: "The MEC notes the tribunal's decision and is taking legal advice."
They could appeal to the High Court against the tribunal's decision, which was itself the result of an appeal against a ruling by the Information Commissioner.
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