Nigel Farage has opened the door to an electoral pact between his UK Independence Party and the Conservatives – but said that Tory MPs would have to oust David Cameron as their leader first.
Some Tory MPs, worried about Ukip's strong performance in last week's local elections, called for a deal between the two parties at the 2015 general election.
It could see Ukip stand aside in seats being fought by Eurosceptic Tories who pledged to vote to leave the EU in the referendum Mr Cameron has promised by 2017.
The Tory leadership dismissed the idea of a pact and there is no sign of a move by Tory MPs to trigger a vote of confidence in Mr Cameron's leadership. However, that could happen if the party panicked after next year's European Parliament elections, when Ukip is expected to come first.
Mr Farage told the BBC's Andrew Marr Show it was "not impossible" that Mr Cameron would be ousted before the general election.
"If he was removed and somebody else was put in place and wanted to come and talk to us and say 'shall we find an accommodation?' we'd consider it, but it is not my priority," said Mr Farage,
Calls for a 2015 pact may grow if Conservative and Ukip councillors form "local coalitions" after last week's elections to keep Labour and the Liberal Democrats out of power in authorities where no party is in overall control.
Peter Bone, a Eurosceptic Tory MP, said Ukip's success made the case for an alliance between the two parties that would see them fielding a joint candidate. "There was a tremendous Conservative vote. There were the conservatives that voted Conservative and the conservatives who voted Ukip. The trick is to get us all together again and that's what we've got to do," he told Sky News.
Mr Cameron is under mounting pressure from Conservative MPs to harden his referendum pledge. Tory ministers may support a backbench Bill to guarantee the public a vote on Europe by 2017. A government Bill will not be included in the Queen's Speech on Wednesday because the Lib Dems will not allow it.
But a Tory MP is expected to bring in a Private Member's Bill and Mr Cameron could back that as a way of putting pressure on Labour and the Lib Dems, who both oppose a referendum.
William Hague, the Foreign Secretary, said yesterday that the Conservatives "will be considering the options" on a referendum. They include two referendums – one before the 2015 election to give Mr Cameron a mandate as he negotiates a "new settlement" with the EU—and another by 2017 on whether to stay in or leave the Union.
David Davis, the former Europe Minister, is leading calls by 20 Tory MPs for a "mandate referendum".
Although the Lib Dems and Labour could block the move, Mr Davis said: "It would be very interesting for the Liberals – and indeed the Labour Party – to vote against giving the public a say on this matter. I think the politics of that for them are very difficult."
Grant Shapps, the Tory chairman, said his party would publish a draft referendum Bill before the 2015 election.
" Of course, we may challenge other parties to support it. If we can get people to support it, then it can come before the Parliament," he said.