David Cameron has been hit by his biggest Tory backbench rebellion yet as the coalition's Lords reform plans cleared their first parliamentary hurdle.
Some 91 Conservatives defied the leadership to oppose giving the legislation its second reading.
Labour's backing for the proposals meant the Government still won comfortably. But earlier rebels had inflicted what could be a mortal blow by forcing the withdrawal of a key timetabling motion - putting the reforms at risk of being "talked out" by opponents.
The scale of the Tory opposition was at the top end of expectations, and illustrates 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 leave their posts rather than support the plans.
The largest previous rebellion came in October last year, when 81 MPs defied a three-line whip to demand a referendum on the European Union membership.
Deputy Prime Minister Nick Clegg sent an email to Liberal Democrat activists hailing the result as "a triumph" for the party. He fired a warning shot to Mr Cameron by insisting he "fully expected" the Conservatives to deliver "this crucial part of the coalition deal - as we have delivered other coalition policies".
"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 to 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 the overall vote had been won by a majority of 338.