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UKIP leader Nigel Farage has demanded a "written in blood" guarantee of a referendum on EU membership before entering any pact with another party.

But he said any future "deal" with the Conservatives would be difficult, as he claims David Cameron opposes a "full, free and fair" vote.

He also insisted his party was not a "pressure group" and demanded action to simplify the tax system.

Mr Farage earlier addressed the party's conference in Birmingham.

He told the 91�ȱ� that he has not "proposed a pact with anybody" and insisted the speculation was coming "from members of the Conservative party who are saying 'we must do a deal with UKIP'".

The UKIP leader said: "I'm making it absolutely clear that we would not do a deal under any circumstances unless the Conservative party... or the Labour party for that matter, were to give an absolute promise, written in blood, that they would give us a full free and fair referendum on our continued membership of the EU.

"It'll need a big change, because Mr Cameron has said on at least half a dozen occasions 'I don't want there to be a referendum on EU membership'.

"They [the Tories] are planning to give us a referendum that says 'do you want to be part of a fully federal Europe or stay in the Union, stay in the single market and the customs union, as we are now?'".

Mr Farage added that he believed voters are beginning to see UKIP as "more relevant" to their lives, and said the party would become the third force in British politics and top the poll at the European elections in 2014.

Addressing party members in Birmingham, he predicted UKIP would cause a political "earthquake" - particularly now that the EU leaders' desire for a fully federal Europe was "out of the bag".

But he vowed not throw away all of the "hard work and sacrifice" the party had made to build up support over the past 20 years to strike an electoral pact with the Conservatives or any other party.

"I am not going to sell this party short over any political deal, there is absolutely no way on earth I would do that," he told activists.

'Flat tax'

Peter Kellner, president of survey organisation YouGov, said he expects UKIP to worry the Tories by potentially wrecking their chances of winning an overall majority.

He said UKIP would "siphon off three, four, five thousand votes in all their marginal seats and that will mean that Labour... and the Liberal Democrats will win more seats, unless they do a deal.

"If the Tories offer [a referendum], I think Labour and the Liberal Democrats will offer it. So the end result of all this might well be a binding referendum on our membership of the European Union soon after the next general election."

Speaking on 91�ȱ� Radio 4, the UKIP leader also discussed other party policies and renewed calls for a single rate "flat tax" and the abolition of national insurance.

Earlier this week, former Conservative peer Lord Stevens joined the party, becoming UKIP's third peer, sitting alongside former party leader Lord Pearson of Rannoch and Lord Willoughby de Broke in Parliament.

Mr Farage also claimed UKIP could spring a surprise in November's police and crime commissioner elections in England and Wales, with the party planning to field candidates in the majority of contests.

The party conference, which runs until Saturday, will also hear from the former Conservative MEP Roger Helmer, who joined UKIP earlier this year, broadcaster Mike Read and journalist Patrick O'Flynn.

The party could face competition for votes from former UKIP MEP Nikki Sinclaire, who has founded the We Demand A Referendum Party - a single-issue party in favour of an in/out referendum on Britain's EU membership - which says it will field candidates in 2014.