It raises doubts about the viability of a deal with the Conservatives

But friends privately say he has all but ruled out agreeing to a referendum

Nick Clegg will deal with whoever emerges on top in a hung parliament

Nick Clegg's 'swivel eyed' obsession with Europe could prevent a second coalition deal with the Conservatives, senior Lib Dems warn

Nick Clegg's 'swivel-eyed' obsession with Europe could prevent a second coalition deal with the Conservatives, senior Liberal Democrats have warned.

The Deputy Prime Minister has publicly said he will consider a deal with whoever emerges on top if the May election results in another hung parliament.

But his friends say that in private he has all but ruled out agreeing to a referendum – raising doubts about the viability of a deal with the Conservatives, and potentially forcing the party into a deal with Labour.

The extraordinary description of Mr Clegg as 'swivel-eyed' underlines the frustration of some senior Lib Dems that Mr Clegg may put his passion for the EU ahead of the interests of the party and country.

One Lib Dem minister said Mr Clegg's obsession was on a par with that of Tory Eurosceptics, who have pushed the Prime Minister to agree to a referendum.

The minister said there was 'no way' Mr Clegg would sign up to the Tory plans, which David Cameron has made a deal-breaker in any coalition talks.

'This is pretty much a red line for Nick,' the source said. 'There is no way he will sign up to plans that will allow the Tories to set the terms of a renegotiation, carry out that renegotiation and then hold a referendum on that basis which could see Britain leave the EU.

'Europe is part of his reason for being in politics – it's his lifeblood. In his own way he's as swivel-eyed about Europe as [Tory Eurosceptic] Peter Bone – it's just that his eyes swivel in the opposite direction.'

An ally of Mr Cameron once famously described Eurosceptic Tory activists as 'swivel-eyed loons'.

The Prime Minister and his Lib Dem deputy attempted to put on a show of unity yesterday with a rare joint press conference to announce a devolution package for Wales.

But behind the scenes, the two men are said to be further apart than ever, particularly over Europe. Many senior Lib Dems believe the party could extract a high price during coalition talks with the Conservatives in return for backing an in-out referendum on Europe.

Mr Clegg served his political apprenticeship in Brussels, working as an aide to former European Commissioner Leon Brittan before becoming an MEP in his own right.

Friends say he was enraged by Mr Cameron's 2011 decision to veto an EU treaty without consulting him, and no longer trusts the Prime Minister on European issues.

For the past two years, Mr Clegg has vetoed attempts by the Conservatives to legislate for a referendum. Mr Cameron – driven by Tory fear of Ukip – has vowed to hold an in/out referendum by the end of 2017 if he wins power in May.

The Prime Minister and Nick Clegg attempted to put on a show of unity yesterday with a rare joint press conference to announce a devolution package for Wales

Speaking to party activists last year he appeared to make it a 'red line' for coalition talks, saying: 'I would not be prime minister of a government unless we could carry out our pledge of an in-out referendum.' One Tory MP said: 'Cameron wouldn't last five minutes if he tried to drop the referendum. He knows that, and we know that. It is not going to happen.'

Meanwhile, Lib Dem foreign affairs spokesman Tim Farron said yesterday that the Lib Dems would have to try to cut a deal with Labour if it ends up as the largest party – even if the Tories get more votes.