Secret estimates of the benefits and risks of identity cards must be published, the freedom of information watchdog has told the government.
The Department of Work and Pensions drew up a report on how the cards could help fight identity fraud.
It refused a Lib Dem request for the report to be published. But Information Commissioner Richard Thomas says its release is in the public interest.
The government has 30 days to decide whether to appeal against his verdict.
Otherwise, the department must release the information under freedom of information laws passed last year.
Critics of the controversial identity cards say the scheme will be hugely expensive and have questioned what benefits it will bring.
They hope Mr Thomas's decision will end a "cover-up" over details of the scheme, which was approved by Parliament earlier this year.
All government departments affected by the scheme have drawn up reports about its long-term benefits.
Three feasibility reports drawn up by Department of Work and Pensions (DWP) officials examined the potential impact on identity fraud - one of the key reasons ministers have used to justify the cards.
They also looked at any possible risks the cards could pose to the department's work.
'Strong public interest'
Lib Dem Mark Oaten asked for a copy of the reports - with sensitive information removed - when he was the party's home affairs spokesman in 2004.
But he was told that making them public could make it harder to get value for money when the government handed out contracts to firms to set up the scheme.
The department also said releasing such information prematurely could stop ministers and officials discussing the pros and cons of policies.
The government lost its case when the decision went to the information watchdog.
In his ruling, Mr Thomas says the information is covered by exemptions to the openness laws - but those are outweighed by the benefits of publishing the reports.
"There is clearly a strong public interest in the public knowing whether the introduction of identity cards will bring benefits to the DWP, and to other government departments, and if so what those benefits will be," he says.
'Right to know'
Mr Thomas says the reports will help informed public debate of the ID card issue - including whether it ought to be compulsory to carry the cards.
"It will allow the public to make a more accurate assessment of whether the significant costs of the scheme are justified by the benefits it is likely to deliver in areas such as the prevention of benefit fraud," says Mr Thomas.
The watchdog has examined the reports and says he can see no information which would put the government's work at risk.
The verdict was welcomed by Lib Dem home affairs spokesman Nick Clegg.
He said: "This decision will hopefully bring to an end the government's attempts to cover up details of the ID cards programme.
"With the cost of ID cards likely to run to billions of pounds, the public has the right to know whether they will be value for money and if the government can really handle a project of this size.
"It is a measure of the government's failure to justify ID cards that during the passage of the bill they never once released a full estimate of its costs and impact.
"Hopefully this decision will lead to a belated revelation of those costs and impacts on the government of this massive scheme."