Tory backbenchers have hit back at Liberal Democrat "blackmail" over House of Lords reform.
Coalition tensions rose again after Nick Clegg's closest aide suggested the Deputy Prime Minister would block changes to parliamentary boundaries if the shake-up of the upper chamber failed.
Richard Reeves, who is stepping down as director of strategy, stressed the importance of next week's vote on a programme motion that would restrict Commons debate of Lords reform. He admitted that without the time limit, the chances of the legislation being passed were "vanishingly small" as opponents would simply talk it out.
Mr Reeves told The Independent that if that happened, the Lib Dems would block plans to cut the number of MPs from 650 to 600 - a measure which some experts believe could give the Tories an additional 20 seats at the next general election.
Mr Reeves said: "It is a very serious moment for the Government. There would be broader consequences for the Government's programme, particularly around political and parliamentary reform. The idea that failure to deliver a Government commitment on Lords reform would be consequence-free is for the birds.
"Anyone who thinks Nick Clegg will shrug his shoulders, say 'never mind' and 'everyone tried our best', will be in for a rude awakening. That is not going to happen."
But despite the tough talk, Mr Reeves said the party would not walk out of the coalition, which would be "crazy", or block Tory public service reforms.
Tory MP Peter Bone said the Lib Dems were acting in an "untrustworthy and unprincipled" fashion.
He said: "They voted for AV and the boundary reform in one Bill. That was the deal. They got the AV referendum in return. If they are really going to do that then Clegg and co should go, and let the Prime Minister govern as a minority Conservative administration. To try to blackmail like this is the lowest."
Mr Bone predicted that there could be more than 100 Conservative rebels in Tuesday's vote on the programme motion. He added: "These are extraordinary numbers in historical terms. Every time it happens it weakens the coalition and it weakens the Prime Minister."