LONDON - David Cameron, leader of the opposition Conservative Party, has left open the possibility of a coalition with the Liberal Democrats by refusing to rule out discussions on reform of Britain's first-past-the-post voting system.

The Tory leader's reluctance to close off the possibility comes amid growing signs that Nick Clegg's Lib Dems, until now the also-rans of British politics, are holding on to their sharply increased support following two televised leaders' debates - and could deprive Cameron's party of an outright majority.

Most opinion polls show the Conservatives ahead as Britain prepares for the general election on May 6.

The Tory leader believes the first-past-the post system is the best for Britain.

"I want us to keep the current system that enables you to throw a government out of office. That is my view," he says.

But when pressed on whether, in the event of a hung parliament, he would be prepared to discuss the Lib Dems' central demand for electoral reform - something he has always opposed until now - he declined to rule it out.

When it was put to him that refusal to move on the issue could mean the Lib Dems teaming up with Labour to push through electoral reform anyway, the Tory leader said: "We think this is an important issue."

Cameron's comments suggest the Tories may be prepared to put reform of the voting system on the table in coalition talks, rather than allow the issue to be a "deal breaker".

After being asked four times to rule out such discussions on electoral reform, Cameron said: "Put the question in, you know, Serbo-Croat if you want to - but you're going to get the same answer."

Labour has promised a referendum on the alternative vote system.

A batch of new polls yesterday pointed again to a hung parliament..

Cameron admitted the campaign had been "shaken up" by "Cleggmania" and said his party had had to change tactics to adopt more positive messages. But he insisted it was still quite possible for the Tories to win outright.

"That's what we are shooting for," he said.

While campaigning in Essex, Cameron attempted to position himself as the champion of radical reform of the political system by saying that prime ministers who took office in the middle of a parliamentary term, as Gordon Brown did in 2007, should be required to secure their own mandate by holding a general election within six months.

Under the plans, a Tory government would amend the act that requires elections to be held at least every five years to ensure parliament was automatically dissolved six months after a change of prime minister for any reason other than a general election.

Lord Peter Mandelson, the Business Secretary, said the idea appeared "to have been thought up over this morning's cornflakes".

He said: "So the Tories would have insisted on an election in 1940?

"This just isn't serious politics."