The government is battling to ensure that estimates of the benefits and risks of identity cards remain secret.
The laws needed to introduce ID cards were passed this year
The freedom of information watchdog ordered the Department of Work and Pensions to publish its findings about how the cards could fight ID fraud.
Now the department has decided to appeal against the information commissioner's ruling.
The Lib Dems say it is disappointing the government is still trying to "cover up the facts about ID cards".
Home affairs spokesman Nick Clegg said: "The public has a right to know if the billions of pounds the government is committing to this massive project will be money well spent.
"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."
The government's appeal will be heard by the Information Tribunal but the process can take several months.
All government departments affected by the controversial ID cards scheme have drawn up reports about its long-term benefits.
Three feasibility reports drawn up by Department for 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.
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.
'Strong public interest'
But the freedom of information watchdog, Information Commissioner Richard Thomas, ruled that the papers should be published.
The benefits of releasing them outweighed the fact that the information was exempt from the full scope of the openness laws, he said.
"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," said Mr Thomas.
He argued the reports would 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.