DAVID Cameron and Nick Clegg are today staring a House of Commons defeat in the face as Tory rebels threaten to wreck their attempts at reforming the House of Lords.

While the Prime Minister has been making a last-ditch attempt to persuade Tory colleagues not to defy the three-line whip in tonight's key vote, it appears only a last-minute concession – on giving the rebels more than the 10 hours proposed to debate the planned shake-up of the second chamber – will stave off the Coalition's first Commons defeat.

Labour leader Ed Miliband has demanded proper scrutiny, saying he wants 20 days for line-by-line analysis of the planned changes.

Lords reform has become the symbol of rising Lib-Con tensions.

Tory rightwingers have complained that reforming the second chamber is simply a sop to the Deputy Prime Minister – enabling him to replace defeated LibDem MPs at the next election with elected LibDem senators, who will hold the balance of power in a revamped House of Lords.

LibDem MPs argue the Coalition Agreement pointed to reforming the upper chamber to make it more modern and democratic.

Tonight's crunch vote will ostensibly be about the so-called programme motion, which seeks to limit the time MPs can debate the Government's proposals to halve the number of peers to 450 and have 80% of the new senators elected by PR.

Yet Tory rebels insist that, for such a major constitutional change, MPs should have full and unrestricted scrutiny.

About 70 Conservative MPs have backed a letter warning that Lords reform threatens to pile a constitutional crisis on top of an economic crisis.

Senior Tories such as Sir Malcolm Rifkind, the former Scottish Secretary, and Nicholas Soames, the former Armed Forces Minister, are among those who have signed the letter.

Given the Coalition has a working majority of 83, a Commons defeat for Mr Cameron and Mr Clegg is looking likely.

No 10 has made clear the usual rules apply so any minister or parliamentary aide who does not vote with the Government will be sacked. Tory MP Conor Burns has made it clear he plans to rebel, even though it would mean losing his Government job. The number of parliamentary private secretaries defying the whips could total five or six.

The Deputy Prime Minister, opening the Commons debate, was heckled and repeatedly interrupted in a noisy chamber as he described the current Lords as a flawed institution.

He told MPs: "We believe the people who make the laws should be chosen by the people subject to those laws."

When Conservative John Redwood suggested the public should be given a vote on the proposed reforms, Mr Clegg replied that a referendum was not justified in this instance.

He said a proposed shake-up had been in the manifestos of all three main parties, a poll would cost £80 million and it would detract attention from a much more important referendum – the one on Scottish independence.

And Sir Malcolm, representing Kensington, argued it would "do far more harm than good to our constitutional structures and to the good government of this country".