David Cameron
The prime minister has offered a vote in 2017 but Ukip want one in 2015 as part of a coalition (Getty)

David Cameron has played down the likelihood of a vote on the UK's membership of the European Union (EU) this year.

The prime minister, who has promised a referendum in 2017, said the chance of a vote would be "pretty slim" after the general election in May.

"I've said there will be a re-negotiation and then a referendum – obviously the sooner that re-negotiation can get done the better and quite frankly the chances of doing that inside 2015 after an election in May is pretty slim," he told ITV's Good Morning Britain.

"I'm going to spend the next 50 odd days campaigning for a Conservative majority government. I think it can be done. If I fall short you can ask me the next day what I'll do about it."

The comments come after Nigel Farage said he would want a vote to be held in 2015 if Ukip went into coalition with the Tories.

"I want a full and fair referendum to be held in 2015 to allow Britons to vote on being in or out of the EU," he said.

Farage also told the Sunday Telegraph that we would negotiate as part of the pact for EU citizens to be barred from voting in the UK.

"It is my strong belief that the four million EU citizens living in the UK without British passports should not be allowed to [vote]," he said.

"And yes, that includes my German wife. They are eligible to vote in European elections, but they should not have the right to decide on Britain's future in the EU. It may be that that would require us to do battle with the European Court of Justice – but so be it."

But the idea of coalition between the two right-wing parties has faced fearsome opposition in some sections of the Tory party.

Conservative grandee and former minister Ken Clarke branded Ukip as "just an angry protest".

"I would be fiercely opposed to anybody doing any deal with a hard-line right-wing nationalist party that wants to blame foreigners and Brussels for all our problems," he told Sky News.

"It would be an extraordinary thing to do to enter into an agreement with a party that is just angry protest."

It is now just 50 days to go before the general election, with the Tories two points ahead of Labour (31% vs 29%) in the latest opinion poll from Tory peer Lord Ashcroft.