Deputy Prime Minister Nick Clegg has accused the Conservatives of engaging in an "unholy bidding war" with the UK Independence Party over who can sound most anti-European, which could end in Britain leaving the European Union.
The Liberal Democrat leader insisted he will not tone down his party's pro-European message, following a poll suggesting his MEPs are set to be wiped out in this year's elections to the European Parliament, while Conservatives slump to third place behind Labour and the UK Independence Party.
Mr Clegg said it would be "economic suicide" for Britain to quit the EU, and that Tories and Ukip were indulging in "silly political games" which could put millions of jobs at risk.
The YouGov survey for The Sun put Labour on 32% and Ukip on 26%, with the Tories trailing on 23% and Liberal Democrats a distant fourth on 9%.
If the findings were repeated on May 22 with a uniform swing, they would see Conservative representation at the European Parliament slashed by 11 to 15 MEPs, while Labour would have 28 (up 15), Ukip 23 (up 10), and the Lib Dems would lose all of their 11 MEPs, according to The Sun's calculations.
Speaking on his Call Clegg show on LBC 97.3 radio, Mr Clegg acknowledged that the Lib Dems were facing a "real uphill struggle" in the European election.
But he added: "Given those kind of predictions, you might have thought that what I'm going to conclude is, 'Let's not talk about Europe too much because making the case for Britain's continued role in Europe is unpopular'. I'm actually going to do exactly the reverse.
"I really genuinely feel that we are getting to the point in the debate where you've got this unholy bidding war between the Conservative Party and Ukip as to who can sound more breathless in their condemnation of all things European and, before you know it, this country will find itself outside of the European Union.
"I think this will mean we will be less relevant and powerful around the world, we would have less influence around the world, but crucially you would have fewer people in work, because being in Europe at the end of the day means being in work. I am not going to stand idly by while people play ever more silly political games and jeopardise millions of jobs in this country.
"Whatever the pollsters say, we are going to remain the party of 'In', because I think being in Europe means being in work."
Voting intentions for the Euro-poll were very different from those for the 2015 general election in the same poll, which put Labour on 39%, Conservatives 33%, Ukip 12% and the Lib Dems 10%.
:: YouGov interviewed 1,893 adults for The Sun on January 14 and 15.
Senior Tory Liam Fox said it was in the national interest to have a debate about Europe, rather than a Conservative response to the electoral threat from Ukip.
He told the BBC Radio 4 Today programme: " I don't think we should be defined politically by anybody else; we need to have this debate because the country needs to have the debate."
Dr Fox said voters would see the European elections as a "referendum on a referendum" and "pretty much anything could happen".
He said: "Anything can happen in elections but I think it's very important that the Conservative Party is defined by the national interest, that we decide that we have to have a rational and reasonable debate about our role in Europe and our role in the world and also Europe's role in the world because o ne of my biggest problems in this debate is not just that it's about Britain's relationship with Europe, it's where does Europe now stand in terms of a very competitive global economy."
He added: "There's only one party in this country that can guarantee the people of this country will get a referendum on the European Union and that is the Conservative Party.
"A vote for anybody else is likely to mean no voice for the British people in their own destiny. That would be a tragedy."
Martin Schulz, the president of the European Parliament, urged the UK to embrace being a "fully-fledged" member of the EU.
He told Today: "The UK is a member of the European Union, so to speak about (the) relationship between the UK and European Union sounds as is the UK would not be a part of the EU.
"It is the UK Government which is a co-legislator in the Council of Ministers, there are 72 members of the UK as lawmakers in the European institution.
"The UK is a part of Europe, so this description 'our relationship with Europe' sounds, for me, strange."
Prime Minister David Cameron has promised to renegotiate the UK's relationship with Brussels before an in/out referendum in 2017.
German MEP Mr Schulz said Mr Cameron had "all the right" to question the relationship " and he does it without interruption".
"Every day he questions it and I have never criticised that he raises questions," Mr Schulz said.
"The other way round, I support him in some of his critical approach to the European Union - a smaller but a more focused European Commission - this is my pledge (for) 20 years.
"But for the challenges we are facing - climate change, worldwide trade relations - we need strong European institutions."
Mr Schulz, seen as a contender to be the next president of the European Commission, said he wanted to see t he "United Kingdom as a fully-fledged member of the European Union, that would be my vision".
He added: "A United Kingdom which is taking part in the development of a common Europe and not speaking about a specific relationship between the European Union and the United Kingdom."