Chancellor Gordon Brown has been ordered to reveal the secret forecasts he was given about the impact of his pensions "grab" in 1997.
Gordon Brown was accused of mounting a £5bn pensions "raid"
Mr Brown was accused of mounting a "raid" on pension funds by removing tax credit on share dividends in 1997.
Now the freedom of information watchdog has told Mr Brown to release Treasury forecasts of the impact of the move.
The Treasury now has until early July to release the papers or appeal against the commissioner's ruling.
It is the second major freedom of information ruling which has gone against the government this month.
The information commissioner's office has also told the Department of Work and Pensions to reveal its estimates of the benefits of the controversial ID cards scheme.
Critics have claimed that Mr Brown wiped billions of pounds off the value of pension funds when he scrapped the tax credit measure.
But Labour has always argued the move removed a distortion in the tax system which encouraged companies to pay out their profits in dividends, rather than reinvest them.
The Treasury was asked last year under freedom of information laws to publish the estimates given to ministers about the loss of revenue to pension funds to be published.
It was also asked to reveal how ministers had considered the impact of the losses, whether they thought about phasing in the changes and how they examined the possible long-term impact on pension funds.
But the Treasury turned down the request, saying officials' advice to ministers was exempt from freedom of information laws.
Now deputy information commissioner Graham Smith has upheld a complaint about the Treasury's refusal to release the documents.
He says the Treasury's concerns are outweighed by the need for transparency in the decision making process.
The Treasury has until 6 July either to release the information or appeal against Mr Smith's decision.
A spokesman would not comment on what the Treasury intended to do.
The Conservatives welcomed the decision.
Shadow chancellor George Osborne said: "Now Gordon Brown will be forced to tell us the truth behind his £5bn pension tax raid, and whether he knew in advance of the damage that he was going to cause to the nation's pensions."
If the papers are published, the public will be able to compare the Treasury's private forecasts with what it said publicly.
In a press notice at the time of the change in 1997, the Treasury said the loss of tax credits would cut the total income of pension schemes and other pension providers by around £3.5bn in a full year.