Conservatives have been trying to stop televised leaders' debates happening ever since it became clear Ukip would be included, Nigel Farage has claimed.

Mr Farage said he does not now believe the debates will take place, but urged broadcasters to "have the guts" to go ahead with the three planned shows and empty-chair David Cameron if the Prime Minister fails to turn up.

Downing Street has made a "final offer" to the broadcasters, which would involve Mr Cameron taking part in a single 90-minute debate involving at least seven party leaders. The Prime Minister accused broadcasters of making a "mess" of the planning for debates and said he was trying to "unblock the logjam".

Labour accused the Prime Minister of "running scared," while Liberal Democrat leader Nick Clegg said he was ready to take Mr Cameron's place in a head-to-head with Ed Miliband.

Mr Farage told LBC radio that Mr Cameron started raising obstacles to the debate - demanding the inclusion of Green leader Natalie Bennett - on the day when media regulator Ofcom proposed that Ukip should be treated as a "major party" in the forthcoming election.

In talks between parties and broadcasters on the proposed debates, Mr Farage said: "It was clear to us from the very first meeting that the intention of David Cameron's team was to destroy the debates and try and stop them from happening at all.

"Why? Firstly, back in 2010, he did come out worst in the debates, and they feared that happening again. Secondly, I think ... that David Cameron is pretty scared of confronting face to face some of the arguments that Ukip would have made."

Mr Farage said that, if the debates went ahead, he would challenge Mr Cameron on the doubling of the national debt under his stewardship and the "open door" to immigration by EU nationals.

"How can you control immigration if you have an open door in Europe to nearly half a billion people? That's the one he doesn't want to confront, that's the one none of them want to confront, and they'd rather it wasn't discussed," he said.

Following Mr Cameron's "final offer", Mr Farage said: "I don't think these debates are going to happen. I hope the broadcasters have the guts to come back and say `We are going ahead as planned with our three debates and Mr Cameron, it's up to you whether you turn up or not'. It would be an empty chair if he chose not to come."

Mr Farage said the broadcasters had so far been "weak" in their response to Mr Cameron's objections, and said it had been "nonsense" for them to decide to include Plaid Cymru and the Scottish National Party in two of the debates, as they do not stand candidates throughout the UK.