Cornwall's six MPs say they have presented a strong case to the Prime Minister, David Cameron, against plans for a joint constituency with Devon.
Mr Cameron met the three Conservatives and three Liberal Democrats on Monday to allow them to voice their opposition to moves to combine the counties.
The government's voting bill includes plans to change constituencies.
St Ives MP Andrew George said Mr Cameron agreed to discuss the issue with ministers.
The Parliamentary Voting and Constituencies 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 MPs - Conservatives George Eustace (Camborne and Redruth), Sheryll Murray (Cornwall South East) and Sarah Newton (Truro and Falmouth); and Lib Dems Mr George, Stephen Gilbert (St Austell and Newquay) and Dan Rogerson (North Cornwall) - said that there were definite economic and cultural differences between Cornwall and Devon.
They added that mixed constituencies might not recognise this.
After the meeting with the prime minister at 10 Downing Street, Mr George said that Mr Cameron agreed to discuss the issue with ministers before it was next debated in the House of Commons next week.
He added that the prime minister demonstrated substantial knowledge of the issue and had been well briefed.
Mr George said: "He said the government would do all it can to ensure that we have the opportunity to debate the issue ... when the bill comes before the House again at its report stage early next week."
The Boundary Commission has until 2013 to make its recommendations about any constituency changes.