David Cameron's EU referendum promise is "not in the national interest" and risks derailing the fragile UK economy, Deputy Prime Minister Nick Clegg warned.

He launched a renewed assault on his coalition partner as several opinion polls suggested the Tories had reaped an opinion poll bounce from the long-awaited policy.

The attack came after Deputy First Minister Nicola Sturgeon had written to EU foreign ministers outlining the role of Scotland within Europe as a "constructive member" and her opposition to Mr Cameron's in/out referendum.

In the letter, Ms Sturgeon highlighted the importance of the European market to Scotland's economy.

She said: "The ability to trade within a single market of 500 million citizens is a central aspect of our strategy to stimulate growth by increasing international trade.

"The European Union continues to be Scotland's top overseas export destination, our exports to the EU are up by around 15% to over £11 billion according to figures."

Mr Cameron insists he wants to stay in the EU but pledged to claw back powers and offer voters a choice of new relationship or exit by the end of 2017 if the Conservatives are returned to power in 2015. It has delighted eurosceptic backbenchers fearful of the challenge of the UK Independence Party.

Mr Clegg dismissed the prospects of securing a significant renegotiation and suggested Mr Cameron should be concentrating on the economy – which risks slumping into a triple-dip recession. My priority will always remain a simple objective of building a stronger economy in a fairer society, enabling everybody to get on in life," the Liberal Democrat leader said. "And I think that job is made more difficult if you have years and years of tying yourself up in knots having arcane debates about the precise terms of the membership of the EU before we get to a referendum.

"It is not in the national interest when we have this fragile recovery, when we have a very open economy which is very dependent on investors in the car industry and the banking system and so on."

Labour leader Ed Miliband defended his decision to oppose the referendum promised by the Prime Minister.

"I just don't think it's in the national interest. Right now it doesn't make sense to commit to a referendum years ahead.

"Has he changed his mind because he's changed his view about the national economic interest? Or has he changed his mind because he's worried about UKIP?"