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."