Exclusive: Reform Lords – or it will cost you 20 MPs, David Cameron told

The Liberal Democrats will block boundary changes that would help the Conservatives' election prospects if Nick Clegg's plan for an elected House of Lords is killed off by a Tory revolt.

In an exclusive interview with The Independent, a senior aide to Mr Clegg warned there would be "consequences" if up to 100 rebel Tory MPs defeat a motion to cut short debate on the House of Lords Reform Bill so it can make progress through Parliament. A knife-edge vote is expected on Tuesday next week.

"It is a very serious moment for the Government," said Richard Reeves, who stood down yesterday as the Deputy Prime Minister's director of strategy. "The vote is hugely significant. It is the critical moment for Lords reform, a once-in-a-generation chance to secure it."

Mr Reeves warned that the Bill's prospects of becoming law would be "vanishingly small" if the timetable motion was lost. Tory opponents of an 80 per cent elected Lords would then make marathon speeches in the hope of killing the measure.

"There would be broader consequences for the Government's programme, particularly around political and parliamentary reform," he said. "The idea that failure to deliver a government commitment on Lords reform would be consequence-free is for the birds." The other main measure in this area will be a Bill to cut the number of MPs from 650 to 600 and bring in new parliamentary boundaries, which experts believe would benefit the Tories by up to 20 seats at the 2015 general election. Tory chiefs are desperate to bring in the new constituencies, which could make the difference between outright victory and a hung parliament.

Mr Reeves suggested the Liberal Democrats would also retaliate if Tory rebels joined with Labour to make the Lords shake-up subject to a referendum. That would scupper Mr Clegg's plan for the first elected peers to be chosen at the 2015 poll. In that event, the Liberal Democrats would delay the boundary changes so they did not take effect until after the 2015 election.

But Mr Reeves said the Liberal Democrats would not walk out of the Coalition, which would be "crazy", or block Tory public service reforms.

The outgoing adviser said the Lords Bill "goes to the heart of what coalition is about – making and keeping deals". He added: "Anyone who thinks Nick Clegg will shrug his shoulders [after a defeat], say 'never mind' and 'everyone tried our best', will be in for a rude awakening. That is not going to happen." Reflecting Mr Clegg's message to David Cameron in their private talks about the Lords vote, Mr Reeves said: "A deal is a deal."

The Liberal Democrats claim there is a Coalition trade-off under which they back the boundary changes, in return for the Tories securing reforms that would see peers elected under proportional representation.

Although Tory whips are working overtime to try to head off a government defeat next week, senior Conservatives deny there is such a deal. They insist the trade-off was between the new boundaries and a referendum on the voting system, which was lost last year.

The Government's majority is at risk next Tuesday because Labour will oppose the timetable motion, even though it supports the Bill. Mr Reeves accused Labour of "acting like teenage boys who want to torture the cat for as long as possible, before killing it."

Mr Reeves admitted that defeat on Mr Clegg's flagship measure would be "damaging" for the Liberal Democrat leader's credibility. He said his party's MPs had had to "hold their noses" in Commons votes for the sake of the Coalition on many more occasions than the Tories.