Nigel Farage has said he was wrong to assume Labour leader Ed Miliband would end up backing a referendum on the UK's EU membership.
The UKIP leader said this was "the one thing I got wrong in this campaign".
- Leave the EU following a referendum ‘out’ vote
- All legislative powers to rest with Westminster
- Negotiate a new trade agreement with the EU
- Legislate for a “lock” that guarantees no transfer of powers from Britain to the EU without an in/out referendum.
- Return Britain to a "leadership role" in a reformed Europe
- Reform the Common Agricultural Policy
- Secure reforms to immigration and welfare rules
Labour has ruled out a referendum, saying it would be a "a clear and present danger" to jobs and business, and Mr Farage said this made a deal impossible between the parties.
David Cameron has promised an in-out referendum by 2017 if he remains PM.
Mr Farage said he did not like the Conservatives' timetable but that there was a possibility of a "conversation" between his party and the Tories.
On Labour's position, he said: "The one thing I got wrong in this campaign is that I said nine months ago that I thought Miliband would go towards the referendum position on Europe to try and box the issue off.
"He didn't do that and from a UKIP voter's perspective it's completely impossible to do a deal with a man that won't give a referendum."
Speaking on the BBC's Question Time election special on Thursday night, Mr Miliband said he did not think it was right to spend two years debating the EU rather than focus on issues like falling wages and the NHS.
With polls pointing towards a hung parliament, Mr Farage has ruled out a formal coalition with any of the parties, but has said he would support a minority Conservative government - although he did not like the timeframe or the terms of the referendum suggested by Mr Cameron.
Lib Dem leader Nick Clegg has said his party would not join any government if it relied on support from UKIP.