The High Court has ruled MPs' expenses should be more transparent
Gordon Brown is to order Labour MPs to back a controversial plan to exempt details of MPs' expenses from the Freedom of Information Act.
The prime minister will impose a three-line whip on the vote, raising the prospect of a backbench rebellion.
No 10 said details of expenses would be more open than ever but critics accuse it of a plot to conceal MPs' expenses.
Lib Dem leader Nick Clegg said the move was "outrageous". Lib Dem and Tory MPs will be ordered to vote against it.
The move follows a long-running Freedom of Information case in which campaigners sought to get details of MPs' expenses, which totalled £87.6m in 2006-7, published.
Last year the High Court ordered the Commons authorities to publish details, including all receipts, to back up claims made by 14 MPs under their second homes allowance. It had been expected that all MPs' expenses details would then be published.
But Commons leader Harriet Harman told MPs last week the government was bringing forward a plan to exempt MPs' expenses from the scope of the FOI Act, on which they will vote on Thursday.
Instead the government says expenses should continue to be published annually, but under 26 categories, rather than the current nine.
Lib Dem leader Mr Clegg confirmed there would be a three-line whip for his MPs to vote against the exemption: "At a time when families are having to count every penny, it is outrageous that MPs are seeking to hide how they spend their money.
"MPs should stop trying to hide their work in secrecy and accept that the same rules that apply to everybody else should also apply to them."
Meanwhile Tory leader David Cameron's spokeswoman said he would be whipping his party to vote against it.
"He is very much against FOI requests being banned for MPs," she said.
"It goes against the principle of transparency."
The prime minister's spokesman said changes to existing rules on expenses would result in a "significant enhancement in transparency".
On Thursday's vote, he confirmed that Labour MPs would be whipped and said the government was acting according to the will of Parliament.
Some Labour MPs have also against the move. Former minister Kate Hoey said it was "crazy" and pledged to vote against the whip.
Harriet Harman says expense claims need to be dealt with "affordably"
"There was no manifesto commitment to oppose transparency. This is very short sighted and just makes the political system look bad."
The first part of Thursday's debate will focus on the FOI exemption and a proposal that expenses will in future be published under 26 headings.
The second part of the debate will focus on revisions to the "Green Book", the Commons rule book on expenses, which includes proposals to allow the National Audit Office greater access to MPs' claims for audit.
An internet campaign by mySociety urging MPs to vote against the change has attracted more than 6,000 signatures. And campaign group Unlock Democracy has taken out a full page advertisement in the Times, describing the move as "shameless".
'Affordable and proportionate'
Campaigners say the proposal would make MPs the only public officials who are immune from public scrutiny and are annoyed that the vote effectively overturns a High Court ruling.
FOI campaigner Heather Brooke told the BBC: "MPs make the law, if they don't like the way the law is, when it comes to themselves, the fact is that they can change it, which is what they've done."
It is thought Commons authorities have already spent hundreds of thousands of pounds scanning and "redacting" receipts dating back to 2005.
Ms Harman told MPs the government's plan would give the public "more information than they ever have before" but in a way that was "affordable and proportionate". She has also said that while receipts would not be published, they would be subject to scrutiny by the NAO.
A spokesman for Ms Harman said on Monday her opponents "were entitled to their point of view" about the effect of the new rule but she would not be responding publicly until Thursday's debate.
The prime minister's spokesman said last week the proposals had support across the Commons and represented a much greater degree of transparency. He also said "security and other" considerations meant MPs should be a special case.