Westcountry MPs last night began the fight to prevent the controversial formation of a constituency straddling Devon and Cornwall.

Sheryll Murray, Conservative MP for South East Cornwall, is to urge the Government to consider Cornwall as a fifth UK country as part of plans to harmonise constituency boundaries.

MPs were last night due to vote on a Bill to reduce the number of MPs from 650 to 600, a move that could mean Devon and Cornwall losing one seat between the counties and prompt a cross-Tamar constituency.

The Parliamentary Voting System and Constituencies Bill also includes plans to hold a referendum on overhauling the electoral system. Cornish nationalists argue that the advent of a "Devonwall" seat would undermine the historic integrity of the Duchy.

Before the vote, most Cornwall MPs – all of whom are from the two coalition parties – indicated their support for the Bill because of the need to either cut the number of MPs or end the first-past-the-post voting system.

But they hope an amendment can be added to the Bill when it is debated next month at committee stage to ensure Cornwall's border remains sacred. If left unaltered, it could mean the rural South East Cornwall seat is fused with Plymouth, or the West Devon and North Cornwall seats are partially merged.

Ahead of the vote, Mrs Murray, whose seat is seen as most at risk of closer cross-Tamar ties, was hoping to make a statement in the Commons about preserving the parliamentary divide.

She said: "A lot of people have a lot of concerns in South East Cornwall about a cross-Tamar constituency. It would be very difficult for a member to serve the constituency (a Plymouth and South East Cornwall seat) to the best of their ability."

She said she will tell Deputy Prime Minister Nick Clegg, who is championing the Bill, that Cornwall has a "unique and specific identity" and that he should consider "Cornwall as a fifth region" alongside England, Scotland, Wales and Northern Ireland. She will also ask the Deputy Prime Minister to apply the same rules to Cornwall as are being applied to Northern Ireland, which she says will be allowed to form constituencies that do not adhere as strictly to the Bill.

She said: "If we fail at parliamentary level to secure an amendment we have a fall back position where we can make representations to the Boundary Commission."

Dan Rogerson, Lib Dem MP for North Cornwall, said losing the Cornish boundary would be "unwelcome".

A referendum on the Alternative Vote system, set to be held on May 5, 2011, was a key concession for the Lib Dems entering the coalition.