MPs will today decide whether to slash the number of MPs from 650 to 600 and make all seats the same size, a move which will see the advent of a controversial "Devonwall" constituency breaching the historic Tamar boundary.
Liberal Democrats have already indicated they will vote against the legislation, in retaliation for Conservatives de-railing reform of the House of Lords.
Ahead of the vote, Cornwall Lib Dem MP Stephen Gilbert said it would be "disappointing to say the least" if Tory MPs from the Duchy refused to join their rebellion.
But Sheryll Murray, a Conservative MP whose Cornish constituency borders Devon, said it is "disingenuous" to vote against the Bill after attempts to secure a Cornwall dispensation failed. Meanwhile, Devon MP Sarah Wollaston warned the Lib Dem revolt would be a "betrayal" to voters and the coalition.
Campaigners for a "whole" Cornwall were last night urging Conservative MPs to back a Lords amendment to the Electoral Registration and Administration Bill, calling for the review to be postponed until well after the next election.
In an email to the three Cornish MPs, Mrs Murray, George Eustice (Camborne and Redruth) and Sarah Newton (Truro and Falmouth), the "Keep Cornwall Whole" group wrote: "We are sure that you will be under some pressure to 'toe the party line' on this matter: we are not asking you to vote against your party, but rather to vote for Cornwall."
Mr Gilbert, Lib Dem MP for St Austell and Newquay, rowed in behind, saying: "When this issue first came up all of Cornwall's MPs stood united and said that we didn't want to see a parliamentary seat crossing the Cornwall-Devon border.
"Indeed, we broke the Government whip and rebelled. It would disappointing to say the least if Conservative MPs in Cornwall, when push comes to shove, aren't prepared to put Cornwall's wider interests before their own narrow party interest and vote against the unwanted 'Devonwall' scheme."
But Mrs Murray, Tory MP for South East Cornwall, said all six Cornwall MPs defied the Government in 2010 by voting for a legislative amendment that would have protected the Duchy from encroachment – but now is the time to move on.
She said: "We've seen referendums in other countries where they hold the vote again and again until they get the right answer. There are so many other things we need to be getting on with that to be re-visiting this now just seems to be silly. I will support the Government and let's get on sorting out the economy. I think it is right that we have 50 fewer MPs, and it is something that will save taxpayers' money. So I think it is disingenuous to be raising that issue now."
Deputy Prime Minster Nick Clegg's decision to order his party to oppose the changes severely hampers the chances of the Tories winning a majority at the next General Election. Without Lib Dem support, the legislation is highly unlikely to be passed – leaving the Conservatives relying on MPs from Northern Ireland for enough support.
Dr Wollaston, MP for Totnes, wrote on The Guardian's website yesterday that the Lib Dem rebellion would be "betraying its claims to champion electoral reform as well as betraying its coalition partners".
She said: "I represent the Totnes constituency which covers the southern end of Torbay and has an electorate of 67,562.
"It cannot be right that the 76,219 voters living in the north of the same bay are relatively disenfranchised.
"Will the Lib Dem MP for Torbay, Adrian Sanders, protest at the injustice that his constituents' votes are 11% less powerful than those of their neighbours in Brixham?"
On micro-blogging site Twitter, Mr Sanders hit back, saying he would have preferred proportional representation rather than "gerrymandering the boundaries".
