Political parties on Cornwall Council have joined forces to urge MPs to thwart Government plans to redraw parliamentary boundaries and create a "Devonwall" constituency straddling both counties.

Yesterday, parties temporarily set aside their political bickering to vote through a motion asking for Cornwall to continue to be represented by MPs whose constituencies lie entirely within the Cornish border.

The motion called for the Parliamentary Voting Systems and Constituencies Bill, currently journeying through Parliament, to ensure the Boundary Commission take note of certain factors when deciding Cornwall's voting future, including geography, local government boundaries and local ties.

At the heart of the new Bill is a bid to cut the number of MPs from 650 to 600 and for all constituencies to have populations within five per cent of the average constituency size.

If left unaltered, Cornwall would end up sharing at least one parliamentary seat with Devon.

On Monday night, the Bill passed its first legislative hurdle after MPs voted in the House of Commons voted in favour of reforming the electoral system, although a number of Westcountry MPs expressed disquiet over the plans. At yesterday's full council meeting at County Hall, Truro, councillors asked that when making a decision the Commission is not "unduly constrained by electoral quotas".

Doris Ansari, leader of the Liberal Democrat group, said: "This motion has cross-party support to keep Cornwall whole as far as constituencies are concerned.

"In the House of Commons debate last night (Monday) it was clear that it's not just us here in Cornwall who are concerned about the proposals of boundary changes.

"We should be speaking to our colleagues in West Devon as I am sure they will be as concerned as we are about cross-border constituencies."

Dick Cole, leader of Mebyon Kernow, said: "A cross-border constituency would have a negative influence on Cornwall's identity and economic direction."

Conservative Armand Toms, said: "Cornwall must be represented in the right way in London. Whether we have five or six MPs – I'm not fussed. "But we must hold that border. I'm glad there's so much support from both sides of the border."

Councillors agreed to send the motion with a covering letter signed by council leaders to the deputy Prime Minister Nick Clegg, to the House of Commons Political and Constitutional Reform Committee and to all UK MPs.