A Lords reform Bill that is set to strain the Coalition to its limits is due to be tabled amid threats of a Tory rebellion and a Labour attempt to derail the timetable for pushing it through.
Labour wants more days allotted to debating the legislation, which would introduce an 80% elected Upper House and slim membership down from 800 to 450, and confirmed it will join with Conservative rebels to vote down a motion setting out its passage through parliament.
Ministers aim to make the Bill law by the spring but a defeat on the timetable would pave the way for as much as four or five weeks of debate in the autumn, which would swallow up time needed by for other business.
Conservative opponents of reform - of whom there are thought to be as many as 100 in the Commons - would also seek to use the opportunity to "talk out" the legislation.
Labour leader Ed Miliband announced his party will back the reforms in the Commons but is expected to table an amendment demanding that any change is subject to a national referendum - something which the Government has firmly ruled out.
The Bill, approved by Cabinet with "strong support" from ministers on Tuesday, 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 July 17.