British voters would be "stark staring bonkers" if they chose to quit the European Union in an upcoming referendum on the UK's membership, Peter Mandelson has warned.
The Labour peer and former business secretary said that Britain would be left "whistling in the wind" in its free trade negotiations if it was no longer part of the European Union.
Countries like India would "just laugh in our faces" and "walk away" from negotiations if the UK chose not to operate as "a bloc with 500 million people behind us", Lord Mandelson argued in a debate at the British Chambers of Commerce' annual conference.
"We'd be stark staring bonkers if we turned our backs on the rich and open markets in the EU and walked out of the market on our doorstep," he added.
Speaking alongside the Labour peer, former Tory chancellor Ken Clarke said that countries in the eurozone would not take the "faintest notice of what the British government's position is" if it was outside of the European Union.
Clarke, who is a minister without portfolio in the coalition, said a British exit would not help "the slightest bit" any attempt to negotiate trade deals with Burma or Brazil.
Clarke poured scorn on the idea of Britain leaving the European Union and adopting a looser relationship based on trade links with the political blocs like Norway or Switzerland, branding it an "absurdity" and a "comic diminution of Britain's role in the world".
Others speaking alongside the pair, like City A.M editor Allister Heath, were more eurosceptic and argued that Britain would still be better off outside of the European Union.
"To me, the mainstream pro-EU case is far too defeatist. Britain's future is global" Heath said.
A post-debate poll of the audience found that two thirds of those present felt that Britain's future resided in staying part of the European Union, while only a third thought the opposite.
He said: "To walk away - or even talking of walking away - from our EU membership would be reckless, foolish and deeply damaging. It would be anti-investment, anti-jobs and anti-business."
Brexiters
Nigel Farage(01 of09)
Open Image ModalThe Ukip leader and MEP is the most famous 'outer'. After his party took over a 100 council seats in May's local election's Nigel Farage is hoping to win the 2014 European elections and then gain MPs in Westminster in 2015. He has confirmed he will seek a parliamentary seat himself. (credit:PA)
Lord Lawson(02 of09)
Open Image ModalMargaret Thatcher's former chancellor and a true 'Tory grandee' revealed in The Times that if and when there is a referendum "I shall be voting out". He also stuck the boot into the David Cameron by saying the prime minister's attempts to renegotiate the terms of the UK's relationship with the EU would be "inconsequential". (credit:PA)
Backbench Brexiters(03 of09)
Open Image ModalThere are quite a few Conservative MPs who would like to wave goodbye to Brussels. Ken Clarke has said the figure is as low as 30 despite the strong eurosceptic feeling on the backbenches. However the exact number is not clear. Mid-Bedforshire MP Nadine Dorries, who remains suspended from the Conservative Party, is currently talk tof the eurosceptic town amid rumours she may defect to Ukip. Other backbench Brexiters include Bill Cash, Douglas Carswell, Peter Bone and Philip Davies and former defence minister Sir Gerald Howarth. (credit:PA)
Labour's 'Let's Leave' Lot(04 of09)
Open Image ModalMost of the anti-EU focus is on the Tory benches. But there are more than a handful of Labour MPs would would like to quit Brussels as well. Eurosceptics include Frank Field, Kate Hoey, Austin Mitchell, and Gisela Stuart.Stuart has argued the status quo is "not sustainable" and Britain should leave. (credit:PA)
Media Moguls(05 of09)
Open Image ModalRupert Murdoch has warned that the EU will "sink" the UK. The News International and boss caused a stir when he met Nigel Farage for dinner in London recently and said the Ukip leader was "reflecting opinion" with his anti-EU views.In November 2010 Richard Desmond’s Daily Express became the first UK newspaper actively to call for Britain to leave the EU, launching a ‘Get Britain Out’ campaign (credit:PA)
I'm A Celeb, Get Us Out Of Here(06 of09)
Open Image ModalOf course no campaign is complete without a bit of star power. The pro-EU camp have Eddie Izzard, who do the Brexiters have?Joan Collins, a 'patron' of Ukip, wants the UK to leave. "The EU, controlled from Brussels, cares only about itself," she said in March. (credit:PA)
The Business Types(07 of09)
Open Image ModalMost business leaders do indeed seem content with what Lawson called the "warm embrace of the European single market", but there are a few dissenters. Private equity guys Jon Moulton and Edmund Truell are two and Next boss and Tory peer Simon Wolfson has said: "Britain should stay in Europe, but only on the right terms". (credit:PA)
The Commentariat(08 of09)
Open Image ModalThere are a number of loud voices whinnying on the sidelines to say "neigh" to the EU notably Melanie Phillips, Richard Littlejohn, Tom Utley, Simon Heffer.Basically the Daily Mail stable. (credit:PA)
Edging Towards Exit(09 of09)
Open Image ModalSeveral high-profile politicians appear to be on the verge of calling for the UK to exit the EU - but just are not there yet.Former defence secretary Liam Fox - pictured here with a big gun - has said "life outside the EU holds no terror" should David Cameron's hopes of negotiating a new treaty fail.Education secretary Michael Gove is said to have told friends the UK has "nothing to be scared of" by leaving Europe.And many other eurosceptic cabinet ministers, including Iain Duncan Smith and Owen Paterson are likely to share that view. (credit:PA)