Westcountry MPs are to lobby Nick Clegg directly after the Deputy Prime Minister hinted that the creation of a constituency straddling the Devon and Cornwall border could be avoided.

Details of a proposed meeting with Mr Clegg emerged as the controversial Bill to overhaul the electoral system cleared its first Commons hurdle.

If approved, the Parliamentary Voting System and Constituencies Bill – central to the Liberal Democrats agreeing to join a coalition Government with the Conservatives – will pave the way for the end of first-past-the-post.

At the same time, the plan could bring about a reduction in the number of MPs from 650 to 600 and equalise constituency sizes.

The Bill has prompted fears in the Westcountry that at least one constituency will have a foot in both Devon and Cornwall, which critics argue would undermine Cornwall's cultural and historic identity.

Mr Clegg, who is championing the Bill, has replied to a letter from Andrew George, Lib Dem MP for St Ives, inviting Cornish MPs to a meeting to discuss their concerns.

In the letter, Mr Clegg said civil servants drawing up the boundaries "may take into account local government boundaries", but that "the principle that constituencies should be of more equal size is paramount".

But offering hope to MPs, he wrote: "It is also worth noting that commissions may also take into account local ties that would be broken by changes in constituencies."

Conservative MPs in the region have also voiced their disquiet at the proposed introduction of the Alternative Vote (AV) system, which ranks parliamentary candidates in order of preference.

Despite the threat of a Tory rebellion, the Government secured a majority of 59 during its second reading.

All of Cornwall's MPs voted in favour of the Bill, with most hoping an amendment can be added that makes a special case of the Duchy.

Before the vote, Gary Streeter, Conservative MP for South West Devon, who abstained from voting and threatened to rebel during later readings, said he feared the move to AV could mean "we would never have an outright Conservative Government again".

He told the House: "Some members say that it is good to have the Lib Dems on board because they will make us much fairer than we would be on our own.

"I find such remarks offensive and absolutely wrong, and I would hate to be party to any measure that would prevent an outright Conservative government from being returned in this country again."

Ben Bradshaw, Labour MP for Exeter, who voted "no", criticised Lib Dem MPs in the region for supporting the Bill. He said: "The plans by the Government to fix constituencies without any consideration of natural or historic boundaries or any proper process of local appeal are completely unacceptable.

"South West Liberal Democrat MPs need to put their money where their mouths are and vote down such outrageous gerrymandering."

If successful, legislation will clear the path for a referendum on AV on May 5, 2011.