Cornwall's MPs are to meet the Prime Minister, David Cameron, over plans for a parliamentary constituency which could be linked with Devon.
The coalition government's Alternative Voting Bill includes plans for changes to constituency boundaries.
The six MPs - three Conservative and three Liberal Democrats - oppose any moves to combine the two counties.
The MPs said they were to ask Mr Cameron next week to intervene to keep Cornwall-only constituencies.
The bill includes plans for an election system referendum, plus plans to "equalise" constituency sizes to about 76,000 people and reduce the number of MPs from 650 to 600.
It is predicted that if boundaries are changed, South East Cornwall or North Cornwall would merge with Devon West and Torridge or Plymouth Moor View, incorporating two sides of the River Tamar.
The Cornish MPs have said that there are definite economic and cultural differences between Cornwall and Devon, and that mixed constituencies might not recognise this.
The MPs - Conservatives George Eustace (Camborne and Redruth), Sheryll Murray (Cornwall South East) and Sarah Newton (Truro and Falmouth); and Lib Dems Andrew George (St Ives), Stephen Gilbert (St Austell and Newquay) and Dan Rogerson (North Cornwall) - are due to meet Mr Cameron in 10 Downing Street on Monday.
The Boundary Commission has until 2013 to make its recommendations about any constituency changes.