Liberal Democrat leader Nick Clegg has warned that he would not prop up a Labour government which had collapsed to third in the popular vote.

He said Britain's "potty" electoral system meant it was possible that Labour could get fewer votes than both the Lib Dems and the Conservatives on May 6 and still emerge as the biggest party in a hung parliament.

In an interview with BBC1's The Andrew Marr Show, he made clear that there was no question in those circumstances of the Lib Dems combining with Labour to enable Gordon Brown to hang on to power.

"It is just preposterous the idea that if a party comes third in the number of votes, it still has somehow the right to carry on squatting in No 10," he said.

"I think a party which has come third - and so millions of people have decided to abandon them - has lost the election spectacularly (and) cannot then lay claim to providing the prime minister of this country."

Mr Clegg said that he believed that whoever formed the next government would have to accept that reform of the electoral system - long a key Lib Dem demand - was now inevitable.

"I think it is unavoidable for any party whatever the outcome. You cannot now duck the fact that we have an electoral system which is completely out of step with the aspirations and hopes of millions of British people," he said.

Mr Clegg sought to reach out to people who supported New Labour in 1997, saying that the Lib Dem programme of promoting fairness and social mobility now offered them a natural home. "That is a liberal progressive agenda whose time I think has come," he said.

Lord Mandelson acknowledged that some former Labour voters were considering "looking elsewhere", but warned them that a vote for the Lib Dems risked a Conservative government.

"You might start flirting with Nick Clegg, but that way you will end up marrying David Cameron," he told the Sunday Mirror. "People have got to remember the biggest number of seats that could change hands are between Tory and Labour. If they vote Lib Dem, they are making it easier for the Tories to get in."

Mr Cameron, meanwhile, poured cold water on the idea that he might be ready to deliver the reform of the Westminster voting system which will be Mr Clegg's prime demand in any talks on a post-election pact.

He told The Observer: "I want us to keep the current system that enables you to throw a government out of office. That is my view ... We think this is an important issue."