A Liberal Democrat Cabinet minister has cast doubt on another coalition with the Conservatives as a senior figure from David Cameron's party admitted it would be "incredibly difficult" to win a majority at the General Election.

Ed Davey warned that it would be "very difficult" to get Lib Dem support for Tory promises to hold an in-out referendum on Britain's European Union membership by 2017.

Mr Davey appeared to align himself with Business Secretary Vince Cable, who has insisted the Lib Dems would not support a referendum, saying the Tory position on Europe is "economic and environmental irresponsibility of the highest order".

The Energy Secretary said Britain could lose its influence in climate change negotiations if at the same time it is attempting to renegotiate the terms of its EU membership - part of the Tory plan before holding a referendum.

With crunch United Nations climate change talks due in Paris later this year, Mr Davey told the Observer: "The Conservative position on Europe is potentially economic and environmental irresponsibility of the highest order.

"Overall our voice in the debate will be dramatically reduced if at the time we are trying to renegotiate our membership of the EU club before a referendum. I am genuinely worried.

"The Tories are putting the national interest behind the party interest. It will be very difficult to get Liberal Democrat agreement to an EU referendum."

The entrenchment of senior Lib Dem figures will make it more difficult for the Tories to form a majority government after May, with former foreign secretary Sir Malcolm Rifkind admitting it would be "incredibly difficult" to win an overall majority.

The comments of the senior Tory, who is standing down at the election, are backed up by the majority of recent opinion polls which show a fracturing of the electorate, suggesting the Lib Dems could again have an influence should they hold on to enough seats.

Sir Malcolm told Sky News's Murnaghan programme: "I think it's going to be incredibly difficult for anyone to win an overall majority because we are in a very different situation.

"You get overall majorities when the two largest parties between them get 80% of the vote.

"That's not going to happen. It's not going to happen in Scotland but it's not going to happen probably in the rest of the kingdom as well.

"I think unless something extraordinary takes place the Conservatives will be the largest party, will continue to form the government, but it won't be a majority one."

Meanwhile, Conservative Health Secretary Jeremy Hunt appeared open to the prospect of reprising the coalition formed in 2010, heaping praise on Lib Dem Care Minister Norman Lamb.

Mr Hunt told the Sunday Times: "I have a very positive working relationship with Norman Lamb.

"He has been incredibly supportive to me in my desire to make the NHS more open and transparent, but I've also supported him in his efforts to improve mental health.

"It has been a very good partnership."

With both Labour and the Conservatives currently looking unlikely to win an outright majority, the smaller parties are set to have an increasingly influential role.

Peter Robinson, leader of the Democratic Unionists (DUP), said the Northern Irish party would talk to either Labour or the Tories but dismissed claims the DUP would demand a £1 billion "ransom" for any future coalition, saying it is "not as vulgar as saying 'here's an amount of money and we want it'".

Northern Ireland's First Minister also warned against allowing the "separatist" Scottish National Party (SNP) into any coalition.

Mr Robinson told the BBC's Andrew Marr Show: "After an election we will sit down with any potential government of the United Kingdom and we'll see to what extent they can help us deliver our plan for Northern Ireland and how we can help in pushing forward their plans for the United Kingdom as a whole.

"So it's not as vulgar as saying 'here's an amount of money and we want it'."

He added: "We certainly don't believe that it will be sensible to allow a lever on government by those who are separatists and want to break up the United Kingdom.

"But in terms of our willingness to talk to the Labour Party as well as the Conservative Party, of course we are."

Labour's election campaign vice chair Lucy Powell insisted though that her party could win a majority.

She told Sky News's Murnaghan programme: "The Tories have given up on a majority government after May 7 - we haven't.

"We still believe that there is every possibility that there could be a Labour majority government and that is what we are fighting for.

"We are doing incredibly well in many of the English and Welsh marginal seats, and the poll out today sort of confirms all of that, and we are sure that we can make sure that we win the seats in Scotland that we need to, that we can win a Labour majority.

"The SNP obviously want to make themselves players in this election, but we are the only party that can beat David Cameron."