A British exit from the European Union (EU) would "impoverish" the continent as a whole, Nick Clegg has warned as he insisted the Conservatives' flirtations with withdrawal will not succeed.
The Deputy Prime Minister said the single market is an incomplete project and more can be done to ensure the EU is seen as part of the solution to the economic problems rather than as the source of them.
Mr Clegg, speaking after bilateral talks with his Irish counterpart Eamon Gilmore, said next year's European elections need to sharpen the focus on increasing the continent's competitiveness as challenges from Asia, Latin America and elsewhere emerge.
Government-backed moves to give voters a say on whether Britain remains in the EU will return to the House of Commons next week, as Tory backbencher James Wharton's private member's bill for a referendum undergoes further debate.
Fellow Conservative MP Adam Afriyie has also warned the Tories risks losing the 2015 election if it declines to bring forward the vote from 2017 to 2014.
Tonight Mr Clegg's talks with Mr Gilmore focused on the EU, the UK and Ireland's trade links and Northern Ireland.
And Liberal Democrat Mr Clegg said: "I can only speak for myself, that's absolutely no agenda to leave and on my part I lead a party which is unambiguously the party of in.
"Of course the European Union needs to be reformed. When I worked in the European Union I remember it took 15 years to decide the definition of chocolate and a chocolate directive. Anything that takes a decade and a half to define what chocolate is in need of reform, much as Whitehall is in need of reform, much as Westminster is in need of reform."
He added: "Reform is one thing, flirting with exit - which is what the Conservative Party appears to be doing - is something that I think if it were ever to happen, which I don't believe it will, would be hugely damaging to the United Kingdom.
"I think it would be damaging to Ireland since we're joined at the hip economically and I think it would also in many ways impoverish the European Union as a whole.
"I don't believe it will happen but we have to win and remake the case constantly that in a footloose, fancy-free world of globalisation where environmental damage knows no borders, criminals don't recognise boundaries, where corporate might can affect one continent to the next, it just makes sense for us in the European Union to do things together that we couldn't possibly do on our own."
Mr Clegg said reforms he would like to see included progress on a single market in energy and digital services.
He added: "There is no surrogate to full membership and I believe not just in full membership of the European Union for the United Kingdom, I believe in wholehearted leadership as well.
"There are a number of areas where the United Kingdom can and always has exercised great leadership - whether it's on foreign defence policy issues, whether on the creation of the single market, whether on the enlargement of the European Union and I will continue to do everything in my power to make sure that we continue to exercise that leadership."
Mr Gilmore said it was in Ireland's national interest for Britain to remain in the EU and at its heart, adding there is room for improvement on how the union does its business.
He said: "As far as Ireland is concerned, it's in our national interest that Britain remains part and parcel of the European Union and very much at its heart. Because as Nick says we have a very close economic relationship and we would like to see a future where we work with Britain on a common and a shared view as to how the European Union should develop.
"I think that's a dialogue that we're certainly very willing to have with the UK."
Of Britain's ongoing EU membership debate, he said: "We're hugely respectful of the fact that this is ultimately a British debate and ultimately a matter for the British people.
"But we have a very clear interest in the issue. We joined the European community as it then was at the same time, we've been over the same ground for a period of 40 years, we've had a very close working relationship with Britain in the European Union.
"Our view is that the European Union has been good for Britain as it has for Ireland but Britain has also been good for the European Union.
"I don't think that anybody should underestimate the contribution that Britain has made to the development of the European Union and the continuing contribution that Britain has to make in the reform of the union and in the advancement of the union's agenda, particularly in the economic sphere."
Speaking immediately after the talks, Mr Clegg explained: "I think we both share the view the single market is, in many respects, an incomplete project.
"There is so much more that we could do to derive more economic benefit, generate more jobs and increase GDP further by completing those parts of the single market that are not complete and to make sure the European Union as a whole is seen as part of the solution to the economic problems that our whole continent faces, not the source of the problems.
"I think particularly as we both look forward to European elections next year the need to really sharpen the focus on increasing Europe's competitiveness as a whole as we face great competitiveness challenges from Asia, Latin America and elsewhere is something we both very much believe is a top priority."