Fundamental divisions among MPs and peers over proposed elections for the Lords are signalling a tough road ahead for the coalition's reforms.
A report published today by parliamentarians who have spent the last six months looking at Nick Clegg's draft Lords reform bill has backed an 80% elected upper House, in which 240 of its 300 members would serve non-renewable terms of 15 years.
It did not achieve unanimity, however, and several key elements of the reform could only be agreed on after votes took place.
"It was the most demanding and wearying committee... that I've had the pleasure and privilege of chairing," committee chair Lord Richard said. He presided over 15 divisions of the committee, reflecting the extent of disagreement over the detail of the proposed changes in Westminster.
Prime minister David Cameron made clear that he supported reform of the House of Lords this morning, after Conservative backbenchers made their opposition to change clear in a meeting of the 1922 committee last week.
"I have always thought that getting an elected element into the House of Lords would actually strengthen our parliament and strengthen the House of Lords and that's the right thing to do," he told the Today programme.
Yesterday deputy prime minister Nick Clegg adopted a more bullish approach, saying politicians should just "get on with it" after 100 years of failed reforms.
He is against a referendum on the issue on the grounds of cost. Last week Mr Clegg told the the Commons' political and constitutional reform committee that a public vote on changing the composition of the upper House would be a waste of public money.
A majority of the MPs and peers looking at the draft bill supported a referendum, however. The committee's report stated: "By any standard, the government's proposal to reform the House of Lords is of major constitutional significance."
Labour has made clear it will only back the changes if the issue is put to the public. Ed Miliband said last week: "I think the best way of making sure that House of Lords reform happens is by giving people a say."
His shadow justice secretary, Sadiq Khan, said he was "heartened" by the committee endorsing Labour's position on the referendum.
He also welcomed its support for a concordat, agreed between the Lords and Commons rather than defined through legislation, defining relations between the two Houses.
"The proposals as currently stand risk total gridlock in the way we are governed, something pro-reformers of all political colours will want to avoid," Mr Khan said.
MPs and peers had also expressed concerns in evidence sessions about how the changes would affect the Commons' primacy over the Lords.
Only a majority of the main committee agreed that existing conventions and legislation like the Parliament Act would be sufficient to ensure the continued dominance of the lower House, however.
A minority report also published today and backed by 12 members of the committee concluded that the government's bill "totally fails" to prevent the Lords threatening the dominance of the Commons.
"We the minority group... believe that the proposals represent an unbridgeable gap between the election of the House of Lords and the primacy of the House of Commons," it stated.
Peers and MPs including constitutional expert Lord Hennessy of Nympsfield called for parliamentary reform to consider both Houses of Parliament rather than just the Lords.
Referendum pressures 'irresistible'
Public approval for the reforms could prove a major sticking point, as questions about the cost of the changes are set to dominate tomorrow's papers.
A YouGov poll conducted for the pro-reform Unlock Democracy group found two-thirds of members of the public support a half, majority or wholly elected second chamber. Only five per cent backed a fully appointed Lords.
Unlock Democracy's director Peter Facey said: "The debate over whether to hold a referendum is a red herring. The question should be whether the public demand one.
"The onus is on the opponents of reform; if they cannot find even five per cent of the public to petition for a referendum, they should not insist the government commits itself to an expensive and needless process."
Meg Russell of UCL's Constitution Unit said that pressure to concede a referendum would become "irresistible" after the recommendations of the joint committee, however.
"The referendum already has support from the Labour leadership and many Conservative backbench MPs," she said.
"If pressed to a vote on the issue in the Commons, the government would almost certainly lose."
Constitutional reform minister Mark Harper, Mr Clegg's Conservative colleague, told The World At One programme that the "burden of proof is with those who want to have a referendum", however.
He dismissed reports that switching to an elected Lords would cost up to £433 million as being "completely speculative".