Deputy Prime Minister Nick Clegg today made clear he expects David Cameron to deliver House of Lords reform, despite suffering his biggest backbench rebellion on the issue last night.
Mr Clegg said the measure - a key priority of his Liberal Democrats following their defeat in the voting reform referendum last year - was a "clear commitment" in the coalition agreement with Conservatives, which he compared to a contract between the two parties.
The House of Lords Reform Bill cleared its first hurdle in the Commons with Labour support last night, but some 91 Conservative MPs defied the leadership to oppose giving the legislation its second reading, while dozens more abstained.
And Mr Cameron ducked a showdown with rebels by ditching a motion which would have limited detailed scrutiny of the reforms to 10 days, leaving open the prospect of opponents wrecking the legislation by dragging out debate until time runs out.
Ministers promised a new timetable motion before the House of Lords Reform Bill enters the committee stage in the autumn, after Mr Clegg warned MPs that without some sort of limit on debate the reform package could be lost.
Speaking to Sky News this morning, Mr Clegg made clear he expects Mr Cameron to use the coming months to bring his party into line to fulfil the commitment he made when the coalition was formed in 2010.
"A deal's a deal and it's important you stick to that deal and you stick to the contract, if you like, that you have entered into," said Mr Clegg.
"That's why I think it is important - not least because so far both parties have stuck to that deal very effectively - that we continue to do so.
"That's why it is important that we deliver House of Lords reform, because it's a clear commitment in the coalition agreement."
Labour's backing for the proposals meant the Government still won the second reading vote with a healthy majority of 338.
But the scale of the Tory mutiny exceeded expectations, illustrating growing frustration among the rank-and-file over the compromises struck by Mr Cameron to govern with the Liberal Democrats.
Two ministerial aides - Conor Burns and Angie Bray - chose to quit their posts rather than support the plans.
The previous largest rebellion came in October last year, when 81 MPs defied a three-line whip to demand a referendum on European Union membership.
It only just fell short of the largest Conservative rebellion since the Second World War, which took place in 1996 when 95 MPs voted against gun control legislation.
There were claims the numbers would have easily topped 100 if the Government had pressed ahead with a division on the timetable motion.
There were reports last night of an angry confrontation between Mr Cameron and one of the rebel ringleaders, Jesse Norman. The premier was said to have approached his MP outside the division lobbies and accused him of not behaving "honourably".
Labour MP Karl Turner wrote on Twitter: "Just witnessed a very angry PM lambasting Tory MP Jesse Norman. Finger pointing and prodding towards Mr Norman."
There were also rumours of a separate incident later where a group of Conservative whips approached Mr Norman in a Commons bar and said he should leave the parliamentary estate for the night.
Meanwhile, Mr Clegg sent an email to activists hailing the result as "a triumph" for the party.
"This evening we overwhelmingly won an historic vote on the Second Reading of the House of Lords Reform Bill - a Bill that will finish something our party started a century ago," Mr Clegg wrote.
"This is a huge triumph for our party, and a clear mandate to deliver much needed reforms to the House of Lords."
He went on: "We have been reasonable and looked at acceptable compromises at every stage. That is why we agreed to withdraw today's timetabling motion, to allow the Conservative team in Government take more time over the summer to talk to their backbench colleagues.
"When we return in the autumn to vote on this again, we fully expect the Conservatives to deliver this crucial part of the coalition deal - as we have delivered other coalition policies."
Tory Cabinet minister Michael Gove said it was a "matter of regret" that so many Tories had chosen to go against the Whip.
But he stressed that overall the vote had been won by a big majority.
"I think it has been a good night for those of us who believe in House of Lords reform, because we have had the biggest vote in favour of House of Lords reform ever," he told the BBC's Newsnight.
"Of course it is a matter of regret to me when I see Conservative colleagues - friends - in a different voting lobby to me.
"There is now a test of for me and others who believe in reform to persuade others."
The division came at the end of two days of debate on the coalition's flagship constitutional shake-up.
Tory backbencher Peter Bone said the vote was a "big step towards a break-up of the coalition".
"There is no way the Prime Minister is going to force this through. It would undermine his leadership.
"The Prime Minister cannot ignore his backbenchers. Some of these people had never rebelled in their lives before."
Ms Bray said she was sad to have lost her job as aide to Cabinet Office minister Francis Maude, but believed the Lords reforms were now "dead".
"I knew that was the consequence," she said. "I am very sorry that I found myself in this position because I have enjoyed working with Francis Maude."
Mr Cameron's official spokesman told reporters at a regular Westminster briefing: "Last night a significant majority of the House of Commons voted to take forward House of Lords reform.
"Clearly there isn't a consensus on the timetable for that, and we are ready to engage with people who support House of Lords reform to find a way forward."
The spokesman's comments appeared to indicate that the Government's efforts over the coming months may be focused on seeking a timetable for debate which would satisfy Labour, rather than trying to win over Tory rebels with concessions in the Bill itself.
Labour was planning to vote against last night's programme motion, which allowed 10 days for line-by-line debate in a committee of the whole House. Party sources said there would be no deal and that ministers should bring the bill to the chamber for a full debate without a timetable.
Mr Cameron's spokesman declined to say whether the Government would offer an extended timetable for debate in the hope of winning Labour support.
But he told reporters: "This isn't an issue that divides on party lines. It is something all major political parties supported at the election. There were 462 MPs who supported the Bill last night.
"We would look to engage with those people to find a way forward.
"If the people who are in favour of reform work together to take forward these reforms, then this legislation will be passed."
The spokesman denied last night's setback meant there was no longer sufficient time for the first tranche of elected peers to take their seats in 2015.
And he rejected descriptions of Mr Cameron's conversation with Mr Norman last night as an "angry exchange", saying the Prime Minister felt "his position had been misrepresented" and "wanted to make clear his position".
The spokesman said Mr Cameron had made clear throughout that he was committed to Lords reform and wanted his party's MPs to observe the three-line whip ordering them to vote in favour, as they would with any other Government bill.
He agreed the PM felt Mr Norman had sought to foster the contrary impression among Tory MPs, that Downing Street would in fact take a soft line on rebels.
A source close to the Prime Minister said the Government would be working hard over the summer to try to find the consensus needed to make progress on House of Lords reform.
"We are trying to achieve Lords reform. We are trying to get this through," said the source.
It is understood that supporters of reform will speak to rebels individually over the coming weeks, though the source acknowledged that some of them would prove "unbudgeable".
But it was unclear whether there would be talks with Labour to strike a deal on the timetable.
It is thought that the Prime Minister is unwilling to see the Government's legislative programme snarled up by unlimited debate on Lords reform, so failure to reach some sort of agreement on the timetable could sound the death knell for the Bill.
Labour leader Ed Miliband sought to capitalise on last night's rebellion when he confronted Mr Cameron at a rowdy session of Prime Minister's Questions in the Commons.
Mr Miliband told the PM that he "didn't just lose the confidence of his party last night, he is losing the confidence of the country".
Describing the coalition as being "in disarray", the Labour leader accused Mr Cameron of losing his temper with Mr Norman, leading to "fisticuffs in the lobby".
Mr Cameron replied that it was "utterly pathetic" for Mr Miliband to tell his MPs to vote in favour of reform, but against the timetable needed to bring it about.
He told Mr Miliband: "If we want to see House of Lords reform, all of those who support House of Lords reform need to not only vote for House of Lords reform but support the means to bring that reform about."
Mr Cameron is expected to make the case for backing Lords reform when he addresses Conservative MPs at a meeting of the backbench 1922 Committee later today.