The Commons anti-sleaze watchdog today called for any decisions on the MPs' expenses controversy to be postponed until after the completion of an independent review.
The cross-party Standards and Privileges Committee urged MPs to oppose Gordon Brown's attempts to resolve some of the issues on Thursday.
Its MPs have tabled a series of amendments which would "defer any conclusions" until after the Committee on Standards in Public Life has had a chance to complete its own inquiry.
A spokesman said the standards committee's members were unanimous in the view that only independent proposals would command public support.
They also felt the Commons should not "pre-empt" the review by the Committee on Standards in Public Life, chaired by Sir Christopher Kelly.
The intervention of the committee, which is widely respected and features MPs from all the main parties, is another serious challenge to the Prime Minister's authority on the issue.
He has already been forced to abandon a proposal to replace the second homes allowance with a per diem rate based on attendance after criticism from all sides of the Commons.
But the Government is proceeding with a debate on Thursday, with a motion proposing interim changes in a number of other areas.
They include scrapping second homes claims for Greater London MPs; requiring MPs to submit receipts to back up all claims; employing MPs' staff centrally; and forcing MPs to declare all outside earnings.
The motion also seeks to agree broad principles for a new second homes allowance that it should be transparent and produce overall cost savings.
But the Standards and Privileges Committee is seeking to override the Government's surviving proposals with amendments to the motion stating that "the House should defer its conclusions until after the Committee has reported".
Its amendments are signed by all 10 MPs who sit on the committee.
Earlier today, former Cabinet minister Charles Clarke said the Prime Minister had been "very" damaged by his handling of the expenses affair and urged him to cancel Thursday's debate.
Mr Brown's hopes of seeking a swift resolution to the expenses row were dashed last night when Sir Christopher made clear his committee would not rush his inquiry.
The Prime Minister asked him to bring forward proposals before MPs break up for their 12-week summer recess on 21 July.
But Sir Christopher let it be known that the committee was "highly unlikely" to have reported by then.
The Standards and Privileges Committee's amendments call for any recommendations to be made "as early as possible", but no decisions should be made until then.
A standards committee spokesman said: "The effect of the amendments is to allow the independent Committee on Standards in Public Life, chaired by Sir Christopher Kelly, to complete its review of members' allowances without being pre-empted by decisions of the House of Commons.
"The members of the (standards) committee are unanimously of the view that only changes arising from an independent, external review of the system of members' allowances will command public support."