The regulatory body set up after the scandal over MPs' expenses has lost a Court of Appeal challenge against an order that it must release copies of receipts submitted by politicians.
The Independent Parliamentary Standards Authority (IPSA) had asked judges to rule on the issue.
IPSA had argued that it was sufficient to disclose a summary of the information contained in receipts.
It follows a Freedom of Information request by a Sunday Telegraph reporter.
During proceedings in March, Master of the Rolls Lord Dyson, sitting with Lord Justice Richards and Lord Justice Ryder, heard that the outcome of the case could have wide and "enduring" consequences, not only for IPSA, but for all other public bodies.
Proceedings were prompted by Ben Leapman, then a Sunday Telegraph journalist, who requested in 2010 that IPSA disclose three specific receipts submitted by MPs in support of expenses claims.
The reporter wanted copies of the originals, but was given a typed-up transcript.
He made a complaint to the Information Commissioner, who ruled in his favour in 2012 and ordered that the receipts be disclosed.
However, following that decision, IPSA failed with two challenges at tribunal hearings.
After dismissing the latest appeal on Tuesday, Lord Justice Richards said the order that copies of the original documents should be released would be put on hold pending any move by IPSA to the Supreme Court.
But an IPSA spokesman said it would "carefully" study the judgment in what was an important test case.
"We were right to test the point of law through an appeal to see whether images of receipts add anything additional to all the information about MPs' expenditure that we already release," he said.
"We remain completely committed to openness and transparency and already publish a detailed breakdown of every claim made by every MP."
ISPA was set up to restore public confidence following the 2009 expenses scandal, which led to the jailing of a number of MPs.
In that same year, parliamentary authorities published more than a million receipts online, but with details such as addresses and account numbers hidden.
In 2010, IPSA decided not to publish receipts routinely, restricting its regular releases to a summary of each claim, saying there was "insufficient public benefit" in publishing receipts and invoices.
In a letter to the Information Commissioner, it said: "Primarily, a trial of extracting and redacting receipts and invoices for the purposes of publication showed that the cost would be in excess of £1m for additional staffing and IT costs alone, compared to the approximate £250,000 cost under the chosen model."
It said "on balance" it took the view that providing images of receipts or invoices would be "disproportionate in terms of costs, insufficiently beneficial in terms of transparency and represented a higher risk in terms of data protection".
But the newspaper argued there was a public interest in viewing the requested receipts in their original form as extra details could be seen which would not be known from a summary.