David Cameron has promised to press ahead with a Commons vote on redrawing the parliamentary boundaries, despite the Liberal Democrats saying they will oppose the plan.
Opposition from the junior coalition Government party, in revenge for Tories blocking House of Lords reform, appeared to scupper plans for a cross-border Devon and Cornwall – or "Devonwall" – constituency.
But the Prime Minister told reporters yesterday: "We want the boundary change vote to go ahead."
Proposals to cut the number of MPs from 650 to 600, while realigning the boundaries to ensure they have roughly equal numbers of voters, would improve Conservative chances of outright victory at the next election, it is claimed.
The Lib Dems withdrew their support for the plan on Monday after Mr Cameron failed to persuade Conservative rebels to back legislation for a partially elected House of Lords.
The Prime Minister said it was clear that Lords reform was not going to succeed. "It became quite clear to me that the Labour Party and others in Parliament were not going to allow Lords reform through," he said.
He added: "I was not going to have months and months of wrangling."
Earlier, however, Westcountry MP and Lib Dem Foreign Office minister Jeremy Browne insisted Lords reform and boundary changes were part of the same "constitutional package" and should both be abandoned.
The Taunton Deane MP said he would vote against the boundary changes, becoming the first Lib Dem minister to confirm he will break collective responsibility, which in normal circumstances would mean being sacked or having to resign from his ministerial post.
A day earlier, Lib Dem leader Nick Clegg accused Tory backbenchers of failing to "honour" the Coalition Agreement which brought the two parties together after the general election.
Mr Browne said: "They're clearly part of the same constitutional package – one of them is about reforming one end of the Houses of Parliament and the other one is about reforming the other end of the Houses of Parliament."
If there is a Commons vote on boundary changes MPs in Cornwall – notably the three Conservatives – will have to choose whether to vote against the plans or for their party.
The Conservatives are highly unlikely to win the vote given Lib Dem and Labour opposition.
Mr Clegg had suggested that, rather than a vote, the legislation is delayed until after the next election in 2015. A full statement by the Prime Minister to the Commons will be expected when Parliament returns next month.
Cornish nationalists have said abandoning the "Devonwall" constituency will safeguard the region's historic boundary, and had called for the boundary review to be "curtailed with immediate effect". The seat would have included Bude in Cornwall and Bideford in Devon.
Andrew George, Liberal Democrat MP for St Ives, said: "The Conservatives were wrong to insist that the Cornish border could not be protected in its pursuit for uniform Parliamentary constituencies.
"I hope all Cornish MPs will vote against the Parliamentary Order when it comes to the Commons next year."
In a statement, the Keep Cornwall Whole campaign group said: "Maintaining Cornwall's territorial integrity and our ancient border is of key importance to Cornwall. Keep Cornwall Whole welcomes and supports the decision taken by the Leader of the Lib Dems and his party to oppose boundary changes. This is a big step forward in removing the threat of Devonwall.
"However, we wait to see how this develops and whether the legislation will be dropped or if it will be deferred until after the election.
"If it is revived during the next Government, Cornwall will meet that challenge again. There is no doubt that delaying legislation often weakens it and this decision by the Deputy Prime Minister means that Cornwall will retain its traditional boundaries for the foreseeable future."