Tony Blair has said it would be "inappropriate" for him to give a view on proposals aimed at exempting MPs from Freedom of Information laws.
MPs are due to debate the bill again on Friday
However, he added that Parliament operated under one of the "most transparent systems in the world".
During prime minister's questions, Lib Dem leader Sir Menzies Campbell said there should not "be one law for MPs and a different law for everyone else".
He said it looked like "Parliament has something to hide".
The Freedom of Information Amendment Bill could be debated on Friday - depending on how long another bill, on building societies lending money, takes to debate.
Opponents of the information bill had thought they had effectively killed it off a week ago when Parliamentary time ran out.
This Friday those opponents are expected to debate at length the financial bill, leaving no time for debate on the information bill and thereby killing it off a second time.
If it is passed, the FOI bill would effectively remove both the Commons and House of Lords from the list of public authorities obliged to release information under the 2000 act, which came into force in 2005.
The bill also protects all MPs' correspondence from release and stops authorities, such as councils or companies, confirming or denying whether they have received a letter from an MP.
Liberal Democrat MP Norman Baker has called the plan "insidious".
He told the BBC it appeared ministers were giving the change their tacit backing.
The bill was introduced following long battles by the House of Commons authorities against being forced to reveal how much elected representatives claimed in travel expenses.
But Tory former chief whip David Maclean, who introduced the bill, said the law was needed to protect constituents and that travel expenses would continue to be published in future.
Mr Blair said at question time on Wednesday: "I think this is a matter for the House and the House can make its view about that.
"Obviously since this is a private members' bill that is before the House, it would not be appropriate for the government to make a commitment one way or another."
Sir Menzies asked: "Do you support this shoddy bill. Yes or no?"
Mr Blair countered: "I am not going to express a view on this."
But he added: "We have one of the most transparent systems in this House and elsewhere anywhere in the world.
"I actually think that occasionally Members of Parliament should stand up for the public service that they give and that they do.
"I happen to think that the majority of Members of Parliament, from whatever political party in this House, actually do a good job of public service, do it in the interests of their constituents, and do it against the background of a more transparent system than most countries have in the world."