Minister Alistair Darling wants tighter restrictions on the Freedom of Information Act, the BBC has learned.
Mr Darling has expressed concerns, the BBC has learnt
The Trade Secretary is concerned that it does not sufficiently protect advice from officials to ministers.
In a letter to the Lord Chancellor he argues that "incremental harm" could be done to policy development and asks for the Act to be reviewed.
There are already moves underway to limit the Act, including one to exempt MPs and another to restrict costs.
Liberal Democrat MP Norman Baker, who has been leading the campaign against exemption, said the leaked letter "blows out of the water any suggestion that the government is neutral" on the issue.
"We never thought they were and this confirms it," he said.
He called on Chancellor Gordon Brown, who refused to block the exemption Bill on Friday, to "come clean about exactly where he stands on open government" and defend the Act.
"We have heard the soundbite, now let's see what he really believes," he said.
But Lord Falconer said the government remained neutral on exemption, because of "considerable support in the Commons" for it.
He said public interest tests - which decide whether an FOI request is answered - were decided by the independent information commissioner and information tribunal and if anyone did not like his decision, they could appeal to an information tribunal.
"We don't want to restrict [the Act], we want to make it better."
In his letter, Mr Darling says there is "a discernible trend within the Information Tribunal that decisions on the public interest test have not been falling in the government's favour in key cases".
He wrote that the demands of the FOI Act were "placing good government at risk".
"If we are to live under constant threat of publication, this will prevent MPs from expressing their views frankly when writing to a minister. We need urgent advice on what the position is," he wrote.
He added the government may have to decide whether to change the law to "redress an apparent imbalance between the 'right to know' and the protection of private space where necessary for good governance".
In March, Treasury documents were released under the Freedom of Information Act which showed officials warned of the effects of abolishing dividend tax credits, saying it "would make a big hole in pensions scheme finances".
It led to claims, denied by the Treasury, that the chancellor had ignored the advice of officials when he abolished tax relief on pensions in his first Budget in 1997.
Among other measures aimed at curbing the FOI Act is a private member's bill brought by Tory MP David Maclean, which would exempt MPs.
Mr Maclean argues it is necessary to protect the confidentiality of correspondence between MPs and constituents.
The government has been accused of covertly supporting the Bill - which critics describe as "deeply hypocritical" - by remaining neutral on it.
But in his letter, Mr Darling, a close political ally of Gordon Brown, writes: "Disclosure of letters between MPs and ministers, even if ostensibly innocuous, will inhibit the dialogue between MPs and their constituents."
Information Commissioner Richard Thomas, in a speech to the FOI conference on Thursday, urged public bodies to adopt a positive approach to openness.
He also said that the government would have to provide "clear, specific and credible evidence that the formulation or development of policy would be materially altered for the worse by the threat of disclosure."
The government is consulting on whether to put more cost limits on FOI requests - which are currently rejected if it costs more than £600 to process them.
Lord Falconer has suggested including the cost of ministers' and officials' time in dealing with requests should be included in that cost.
But Campaign for Freedom of Information director Maurice Frankel has suggested it would make it easier to bury potentially embarrassing requests.