David Cameron has said Conservative peers will block the Freedom of Information (FoI) exemption bill in its current form - even though a former Tory whip tabled the original private member's bill.
The Conservative leader has acknowledged there are concerns with proposals to exempt themselves from the FoI act and said peers will not support it in its current form.
It is thought peers could allow correspondence between MPs and their constituents to be kept private, but insist members' expenses are made freely available.
Speaking at a news conference in London, Mr Cameron said: "I think it is not really enough to say that this is just a House of Commons matter now because I think that there is a big debate about MPs making themselves not subject to the Freedom of Information Act when others are."
The Liberal Democrats, who today launched a petition against the exemption, criticised Mr Cameron's delayed stance on the bill, which has already passed through the House of Commons.
"It is typical of David Cameron to wait and see what the newspaper editorials say before he makes up his mind on an issue," said Lib Dem leader of the house David Heath.
He added: "Perhaps he should be doing fewer staged photo calls and paying more attention to what his party is actually doing."
Liberal Democrat leader Menzies Campbell had earlier called on Mr Cameron, along with Gordon Brown, to publicly denounce the bill.
Mr Brown has not commented on the bill, but it is suspected the government is covertly supporting the proposal - even though it provides an exemption from its own policy.
Ed Balls, who is a long-term ally to Mr Brown, has suggested the bill could be passed as long as MPs' expenses are not included.
Mr Brown launched his campaign for Labour leadership with a pledge to foster a more open and transparent form of government.
However, Edward Davey, chief of staff to Sir Menzies, warned the exemption will only serve to reinforce the public perception of parliament as opaque and clouded in secrecy.
"If Gordon Brown is serious about reforming our democracy this is his first real test," Mr Davey said.