Clegg: "A party that comes third... cannot lay claim to providing the PM"
It would be "preposterous" for Labour to have its leader remain in Number 10 if it came third in terms of votes, says Lib Dem leader Nick Clegg.
He told the BBC it was "potty" Labour could get fewer votes than their rivals but still try to form a government.
His comments come as four opinion polls point to a hung parliament.
Tory leader David Cameron said that would lead to "bickering". Gordon Brown said he would not "assume an outcome" but "accept the people's verdict".
A hung parliament is one in which no party has an overall majority - that is, more than half of the MPs in the House of Commons.
A party can stay in power by trying to forge an alliance with a smaller party to create a coalition government or the biggest party may form a minority government and try to form majorities over individual bills.
Up until today Nick Clegg has been very very careful to stick to a carefully drawn-up formula about how the Liberal Democrats would react to a hung parliament
In such a scenario, the existing prime minister remains in power until he or she resigns, and may try to stay on even if his or her party does not win the largest number of seats.
Sunday's opinion polls all suggested the Conservatives would win the most seats but still fall short of a majority.
Speaking on the BBC's Andrew Marr Show, Mr Clegg referred to the fact that three of the polls also indicated Labour might poll fewer votes than the Lib Dems yet still secure more seats.
He said: "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.
"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 defended his party's policy of offering an amnesty for long-term illegal immigrants as a "one-off solution to a one-off problem". Strict checks would ensure they were not criminals, spoke English and would pay tax, he said.
The Lib Dem leader said whoever formed the next government would have to accept that reform of the electoral system - one of his party's key demands - was inevitable, and that any Tory opposition to electoral reform would be a "massive strategic error".
And he spoke for the first time about the possibility of sitting in cabinet with rival parties, after the general election.
However, when asked if he would be happy to remain as prime minister should Labour have only the third-highest number of votes, Mr Brown said: "I don't think that's going to happen.
"When the electorate make up their mind we've got to accept the verdict."
In an interview with Dermot O'Leary, to be screened on BBC3 on Wednesday, he said it would be "arrogant" to make assumptions about the situation after the election.
Home Secretary Alan Johnson told the BBC it would be a "difficult" situation, should Labour finish third in terms of the popular vote.
"Look, it doesn't take a genius to think that if you are third in the popular vote, then... you are not best-placed to deal with it," he said.
Alan Johnson says being third would make things "more difficult"
"We want to be first in the popular vote. We are going out over the next 11 days - and there is an awful lot of uncommitted voters."
Mr Johnson insisted his assertion that "after the election there will be a debate on proportional representation on the electoral system" was not "waving the flag" at the Lib Dems to say he would be prepared to do a deal with them to secure a coalition.
"It is empowering the voters that matters," he said, adding that Mr Brown recognised the need for "dramatic change" to the constitution.
Mr Cameron, meanwhile, repeated his assertion that a hung parliament would lead to weak government, saying it could lead to "bickering, argument, drift and dithering".
He added that he was against proportional representation, although a Conservative source told the BBC if voters returned a hung parliament he would be "constructive". That could pave the way for a coalition agreement between the parties.
Mr Cameron said: "My fear is... we'll all potter down to the polling station, we'll all put a cross in the box, we'll think we've changed our government.
"We'll wake up the next day and find that Tweedle Dum is talking to Tweedle Dee, who's talking to Tweedle Dem and they're all jumbled up again into a new government with the same people in the same places. They'd be picking the prime minister, not you."
The BBC's Nick Robinson said Mr Clegg saying Labour could not govern if they had the most seats but the least votes marked a shift in his position, as previously he has said the party with the biggest mandate should get the chance to govern.
The Conservatives unveiled their latest poster campaign, comprising 2,200 billboard advertisements showing the leader along with slogans such as "Let's scrap ID cards" and "Let's fund new NHS cancer drugs".
David Cameron: ''Hung parliaments, hung councils, don't get things done''
Meanwhile, more than 45 TV stars, including Harry Enfield, Jo Brand, Eddie Izzard and former Dr Who star David Tennant, signed a letter to Sunday's Observer criticising Tory plans for the BBC.
However, a party spokesman said the Conservatives had "continually expressed our support for a strong and independent BBC".
Editor of the BBC's political research unit, David Cowling, said the latest polls for Sunday's newspapers suggested the Conservatives could narrowly nudge ahead of Labour as the largest single party in the Commons but still not secure an overall majority.
A ComRes poll of 1,006 adults on Friday and Saturday for The Independent on Sunday and Sunday Mirror showed the Tories on 34% - up three points on last Sunday's poll - the Lib Dems in second place with 29% (no change) and Labour third on 28% (up one).
For the Sunday Times, a YouGov poll of 1,412 voters conducted on the same days put the Conservatives up two points on last week at 35%, the Lib Dems down one on 28% and Labour down three on 27%.
Meanwhile Ipsos-Mori - for the News of the World - put the Tories up four points on 36%, Labour up two on 30% and the Lib Dems down nine points since Monday, on 23%. It surveyed 1,245 adults on Friday.
An ICM survey for the Sunday Telegraph put the Tories on 35% (up two points on last Sunday), Lib Dems on 31% (up one) and Labour on 26% (down two).
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