Nick Clegg, the leader of Britain's Liberal Democrats, has warned Labour he will not prop up PM Gordon Brown in the event of a hung parliament.
Clegg's party has surged to second place in the opinion polls after he performed strongly in two recent TV debates.
The Lib Dems are unlikely to win enough votes to form a government after voters go to the polls on May 6, but they could hold the balance of power in a hung parliament.
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That would leave Clegg and his party to choose between Labour and the Conservatives, potentially teaming up with one or the other to govern.
The Lib Dems have never said which party they would work with in such circumstances - although Clegg has said the party with the strongest "mandate" should be allowed a shot at government.
But in a clear message to Brown, Clegg has now warned the premier's party - in power for 13 years - that he would not support them if they dropped to third place in the general election.
No outright majority: Polls
"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 Number Ten (Downing Street)," he told the BBC.
"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."
Clegg spoke as opinion polls reinforced the trend of recent weeks, showing the centre-right Conservatives in the lead but without enough support to win an outright majority.
A YouGov survey in the Sunday Times put the Tories on 35 per cent, a rise of two points, with the Liberal Democrats down one on 28 per cent and Labour down three on 27 per cent.
A YouGov poll in Monday's edition of The Sun - which is publishing a survey every day in the run-up to the election - showed the Tories unchanged on 34 per cent, the Lib Dems up one on 30 per cent and Labour down one on 28 per cent.
Conservative leader David Cameron said he would not support the Liberal Democrats' top demand - reforming the voting system - sending a signal that the two parties would find it hard to work together, although he did not rule that out.
Voting system reform plea
"I want us to keep the current system that enables you to throw a government out of office. That is my view," he told the Observer newspaper.
Home Secretary Alan Johnson meanwhile told the BBC that "after the election there will be a debate on proportional representation on the electoral system," sparking speculation that he was making an overture to the Lib Dems.
But Johnson - who has been touted as a possible successor to Brown if Labour lose the election - insisted he was not "waving the flag" at the Lib Dems to signal he would be prepared to do a deal on voting reform to secure a coalition government.
Meanwhile, Business Secretary Peter Mandelson, seen as Brown's de facto deputy, has warned supporters of centre-left Labour who were thinking of switching to the Liberal Democrats that doing so could let in Cameron.
"You might start flirting with Nick Clegg, but that way you will end up marrying David Cameron," he told the Sunday Mirror.
Disillusioned Labour voters in Britain sometimes back the Liberal Democrats in protest as both are seen as progressive parties.
