MPs vote on the Wright proposals later this month
Plans to beef up the powers of backbench MPs may not become law before a general election, Prime Minister Gordon Brown has said.
Mr Brown urged MPs of all parties to back the plan in a vote on 23 February.
But he also said the architect of the reforms, Labour's Tony Wright, had to be "realistic" about the prospect of them being passed in time.
The changes include elected select committee chairmen and handing control of what MPs debate to backbenchers.
The plans also include trialling a petitions committee, that could allow the public to select the subject of debates in the Commons.
Mr Wright's proposals were published in November as part of efforts to clean up politics in the wake of the expenses scandal.
Gordon Brown has given them enthusiastic backing in the Commons but he stopped short of guaranteeing parliamentary time when challenged by Mr Wright.
Mr Wright said the the government had been accused of coming up with a "rather clever way of sinking the whole subject" after they had set up a "non-amendable" vote which would allow the proposals to be sunk by an objection from just one MP.
The Labour MP called on Mr Brown to guarantee that if the proposals were rejected in this way, as seemed likely, they would be brought back to the Commons "immediately".
Mr Brown replied: "Yes, we would bring the proposal back but let's be realistic - if we have to go through line-by-line, dot and comma, on each of these proposals then we will not have the parliamentary time to be able do that.
"What we are really trying to say is - we prefer there to be progress as quickly as possible. We prefer there to be progress by consensus. If we can get an agreement by all parties and all members of the house that this is the right way to go forward then that is something that these major proposals can be agreed as soon as possible.
"But if it so happens that people object, and I wish they won't, because I want them to go forward, then we will have to start looking at this line-by-line."
Last week Commons leader Harriet Harman said there would be a full day's debate on the 21 proposals, which she said would cover four "big ticket areas that we want to make change on" and which she believed MPs had a "good prospect of achieving a consensus".
But she also revealed that the liaison committee itself is split on one of the key proposals, the election of select committee chairman, suggesting consensus might be difficult to achieve.
A coalition of parliamentary and constitutional reform organisations has urged the government and MPs to implement the reforms before a general election.
The Better Government Initiative, Constitution Unit, Democratic Audit, Electoral Reform Society, Hansard Society, Power 2010 and Unlock Democracy all back the Wright reforms.
Ruth Fox, of the Hansard Society, said: "This parliament more than any other in recent memory needs to reform itself.
"The reputation of MPs, of parliament and that of the government itself will be further eroded if, having established a clear direction for reform, a procedural roadblock is now erected and the reforms are blocked before the general election."