Ed Balls has warned that a British exit from the European Union would be "reckless, foolish and deeply damaging".
Speaking at the CBI's Annual Conference on Monday, the Shadow Chancellor said: "Europe needs reform, but to walk away from our EU membership would be reckless, foolish and deeply damaging. On Britain’s future in Europe, the national interest must come first."
Balls mocked Prime Minister David Cameron's pledge to hold a referendum on Britain's EU membership in 2017, saying that it was motivated by "internal party management".
He added: "Britain is better placed to shape Europe’s future if we are fully engaged rather than having one foot out the door."
“Some on the extremes of left and right see the solution to be isolationism, turning inwards, setting their face against the rest of the world. They are wrong. Business is key to the solution - as is Britain properly engaged in a reformed Europe."
Balls' warning on Britain's place in the European Union comes as a new report from the CBI suggested that staying in the EU was "overwhelmingly" beneficial.
The CBI said business also wanted a moratorium on legislation which could be made at national level, and a permanent UK opt-out from the Working Time Directive.
Director general John Cridland said: "We have looked beyond the political rhetoric to examine the pros and cons of EU membership and British business is unequivocal - the Single Market is fundamental to our future.
"We are better off in a reformed EU than outside with no influence. Each year, membership is worth £3,000 to every household in this country.
"But the EU isn't perfect and there is a growing unease about the creeping extension of EU authority. Europe has to become more open, competitive and outward-looking if we are to grow and create opportunities and jobs for all our citizens.
"Contrary to popular myth, the UK is influential in the corridors of Brussels and will still be as long as we play our cards right. The Single Market is a great British success story and the best way for us to remain a leader on the world stage is from within the EU."
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)