Media playback is unsupported on your device Media caption Nick Clegg: "Leaving the European Union would be economic suicide. You cannot overstate the damage it would do"

It would be "economic suicide" for Britain to leave the European Union, Nick Clegg has warned.

The deputy PM said a new "coalition in the national interest" was needed to make the case for membership, and asked businesses and charities to help.

He attacked David Cameron's proposed referendum, calling it a "short-sighted political calculation" which could see the UK "stumble out" of the EU.

Eurosceptic group Business for Britain accused Mr Clegg of "scaremongering".

In a speech at the London headquarters of Swiss technology firm Buhler Group, Mr Clegg said EU withdrawal would leave Britain "isolated and irrelevant in our own backyard" and not "taken seriously" by world superpowers.

'Ludicrous myth-making'

He has written to 100 businesses, charities and trade groups urging them to speak out about the benefits the EU brings.

"My great fear, in all of this, is that pro-Europeans are being too slow to wake up to the danger ahead," the Liberal Democrat leader said.

"The day I dread - the day I hope never comes - is a time when it is all too late: Britain has stumbled out of the EU, and we look back to these days and say we should have done more.

"It will not be enough to speak up on the eve of a referendum. We need to start challenging some of the ludicrous myth-making by the isolationists now."

Mr Clegg said he accepted a referendum was inevitable but believed it should be held after the change to EU rules.

Mr Cameron has promised a public vote on EU membership in 2017 if the Conservatives win the next general election.

But Mr Clegg said that was designed to "plaster over" internal Tory divisions over Europe until after the 2015 general election.

Mr Clegg said: "Next May the Euro elections are bound to become a proxy for the bigger questions of 'in versus out' - a debate that will play out in the 2015 general election, too."

'Semi-detached'

He argued that "you cannot overstate the damage" that leaving the EU would do and predict that having to negotiate individual trade treaties would take a decade.

"We simply will not be taken seriously by the Americans, the Chinese, the Indians, all the big superpowers, if we're isolated and irrelevant in our own backyard," he said.

On Sunday Conservative backbench MP Adam Afriyie said he would try to force the government to hold the referendum in 2014, and Mr Clegg said it was "not even clear" Mr Cameron would be able to delay the vote until 2017.

He also questioned whether any reform of Britain's relationship with the EU would "ever be acceptable to the large swathes in the Conservative Party who want to be semi-detached from the EU or out of it altogether".

Business for Britain chief executive Matthew Elliott said: "Nick Clegg has shown today that he is out of touch with how the majority of the British people, including thousands of British business people, feel about the European Union."

He added: "Contrary to what he said in this speech, it is the deputy prime minister rather than David Cameron who is 'playing with fire'. By writing off the process of renegotiation before it has even properly begun, Nick Clegg is increasing the likelihood of Britain voting to leave."

But TUC general secretary Frances O'Grady said: "Nick Clegg is right to make the case for the EU and say that paid holidays and other workplace rights are some of the best arguments for EU membership. Leaving the EU would be disastrous for jobs and investment."

Separately, in a Westminster debate on UK-US relations, Conservative MP Michael Fabricant said: "The United States is very keen that we remain in the European Union but that is partly because some US diplomats have said privately they think the United Kingdom is the only sane voice in the European Union when it comes to some issues."