UK Independence Party (Ukip) leader Nigel Farage today said he was ready to do an electoral deal with the Conservatives in return for a promise “written in blood” of an in/out referendum on British membership of the European Union (EU).
Mr Farage denied reports that he was offering the Tories a deal under which Ukip would give Conservative candidates a clear run at the 2015 election in return for a referendum pledge, and promised members that he would not "sell short" the party.
But in his keynote speech to the Ukip annual conference in Birmingham, he said it would be "silly" of Ukip not to consider any offers made by the Conservatives - or even Labour.
He predicted that Deputy Prime Minister Nick Clegg would be appointed a European Commissioner ahead of the election, leaving the Tories facing a possible coalition between Labour and the Liberal Democrats under a more left-wing leader like Vince Cable or Tim Farron, and in need of allies.
Mr Farage told Ukip delegates: "If an opportunity came which meant that we could get this country closer to walking through a door marked 'UK independence', if we had an opportunity to do something that was in our national interest, it would be silly not at least to consider it.
"Is that going to happen? I don't know. As the general election approaches, people will be looking for supporters. It could be Labour - more likely, perhaps, it will be the Conservatives.
"But all this talk of a deal with the Conservatives hasn't come from me. It has come from members of the Conservative Party saying to David Cameron 'Look, the Lib Dems are going to get rid of Clegg, because you are going to make him the European Commissioner in November 2014 to replace Baroness Ashton, and the Lib Dems under Vince Cable or Tim Farron will look to do a deal with Labour, and if Ukip are on 10% or more in the polls, you must look to do a deal with Ukip'.
"That's where this whole supposed negotiation comes from."
Mr Farage insisted that Ukip will remain an independent and distinctive political party, fighting elections for police commissioners later this year, as well as parliamentary by-elections, county council elections and the 2014 European Parliament elections, when observers believe the eurosceptic party could finish second or even first.
He said that he might not even be willing to accept the promise of a referendum, recalling that Mr Cameron offered a "cast-iron guarantee" of a national vote on the Lisbon Treaty before the 2010 election.
He said: "Having spent nearly 20 years of hard work, sacrifice and much mockery building Ukip into a political brand in this country that is now connecting with millions of ordinary men and women out there, you have my absolute assurance that I am not going to sell this party short for any political gain - there's absolutely no way on earth that I would do that.
"The only way that we would even consider a negotiation of any kind at all would be if an absolute promise was made to give this country a full, free and fair referendum, so we could decide whether we remain members of the EU or not - that would have to be on the table before we would even consider any proposal and we would possibly have a problem even then.
"There are one or two people in politics who make promises and then break them, so I don't think a cast-iron guarantee would satisfy Ukip. I think, at a minimum, it would have to be written in blood."
Mr Cameron did not call a referendum on the Lisbon Treaty, as it had been ratified by the time the coalition Government came to power.
But he has legislated to require a public vote on any future EU treaty which transfers power from Westminster to Brussels.
Mr Farage said that it was almost inevitable that a referendum would be held if the EU pushed ahead with recent proposals from European Commission president Jose Manuel Barroso for a federation of nation states.
But he said that this would produce "the wrong referendum", as it would not include the option of the UK leaving the EU altogether.