The House of Commons could be forced to release a more detailed breakdown of MPs' travel expenses after losing a Freedom of Information case.
MPs' expenses are only set out under broad headings
The Information Tribunal rejected two appeals by Commons authorities that revealing further details would breach data protection principles.
MPs' expenses are published each year, but only under broad headings.
Liberal Democrat MP Norman Baker, who made one of the FOI requests, said he was "delighted" with the ruling.
Figures released last October revealed that MPs had claimed almost £86.8m in expenses and allowances during the previous year, an increase of nearly £6m on the previous year.
The 2005/06 payments - about £131,000 per MP - come on top of a basic salary of £59,686 and a pension.
Mr Baker, MP for Lewes, said: "There is absolutely no reason not to release this information to the public.
"I am the first to defend people's right to privacy but I do not think this information is personal in the least.
"This information will simply let people know much MPs are spending on different modes of travel. If the public are paying then they have the right to know what the money is being spent on."
He added: "Let us see which MPs are caring for the environment by using public transport around town and which are still happily driving everywhere despite available alternatives."
The House of Commons must now release the information within 30 days, or appeal to the High Court on a point of law.