Parliament is braced for a lengthy pitched battle over reform of the House of Lords after Labour confirmed it will combine with Conservative rebels to vote down the government's timetable for legislation.
A Bill to introduce an 80% elected Upper House, slimmed down from 800 to 450 members, will be tabled on Wednesday and ministers aim to make it law by the spring - using the Parliament Acts if necessary to override expected opposition from peers.
Labour leader Ed Miliband announced his party will back the reforms in the Commons but accused the government of failing to provide enough time to debate them properly.
His MPs will join Tory rebels in opposing the timetable, potentially paving the way for as much as four or five weeks of debate in the autumn which would swallow up time needed by ministers for other business.
If the government's timetable is defeated, Conservative opponents of reform - of whom there are thought to be as many as 100 in the Commons - will seek to use the opportunity to "talk out" the legislation.
Meanwhile, Labour is expected to table an amendment demanding that any change is subject to a national referendum - something which the government has firmly ruled out, citing the estimated £100 million-plus cost of staging a vote on reforms which were promised by all three parties in their election manifestos.
The Bill, approved by Cabinet with "strong support" from ministers, is being driven by deputy prime minister Nick Clegg and forms the remaining centrepiece of Liberal Democrat constitutional reform plans, following defeat in last year's referendum on voting reform.
It would finally complete the removal of hereditary peers from the Second Chamber and introduce the first elected members in tranches of 120 at each of the next three general elections, with the process of change completed by 2025. Elected members would serve for a single 15-year term.
In a concession to critics, ministers have scrapped plans for a salary of about £60,000 for members of the new Upper House.
Members will instead receive £300 for each day they attend - a maximum of about £45,000 a year - and this sum will be taxed, unlike the attendance allowances currently paid to peers.
Ministers insist that the reforms will maintain the primacy of the House of Commons within Parliament. But critics warn that this will be under threat once the Upper House has the added clout of democratic legitimacy.
After publication, the Bill will have its second reading in the Commons, followed by the crucial vote on the timetable motion before Parliament rises for its summer break on 17 July.
Prime minister David Cameron's official spokesman said that Conservative MPs would be whipped "appropriately" to support the legislation. But he declined to go into detail about whipping arrangements, sparking speculation that they may escape significant disciplinary action for breaking ranks.
Penny Mordaunt, a member of the executive of the backbench 1922 Committee, said she was among many Tory MPs who have "grave reservations" and may rebel.
"Many of my colleagues who have been in the House much longer than I have and have never voted against the government are considering doing so," she told BBC Radio 4's World At One.
"This is a very serious issue and indeed there are many people on the government payroll who have grave concerns about this...
"If we want to reform them, we should at least pause and work out what it is we are trying to do, rather than trash the constitution, which is what these reforms are going to do."
Mr Miliband denied he was seeking to wreck the legislation.
But he said that as it stood, the Bill contained a number of "flaws" - including the absence of a referendum - which Labour will seek to rectify through amendments in the Commons.
"I do not want the reform of the House of Lords to be stuck in the House of Commons," said the Labour leader.
"I want a good reform Bill to get out the Commons and into the Lords so it can be properly discussed in both Houses."
Senior Labour sources declined to say how many days they believe are needed to discuss the reforms in the Commons, but pointed to the examples of the debate on the Maastricht Treaty, which took up 23 days, and the Lisbon Treaty, which lasted for 11.
Labour MPs will be whipped to support the reforms but Mr Miliband is expecting some rebellions on his side. Sources said "a wide and diverse range of views" were expressed on the issue at a meeting of the parliamentary party.
Liberal Democrats responded with scorn to Mr Miliband's stance.
A senior Lib Dem source said: "I don't think the public will understand them voting for this in one bit and against it in the other. As a supposedly progressive party, they should support this reform.
"They need to come out and justify why they think spending over £100 million on a referendum is a good idea when the public don't show a huge appetite for it and it was in all three party manifestos."
Suggested For You
Get top stories and blog posts emailed to me each day. Newsletters may offer personalized content or advertisements. Learn more