Ed Balls has warned that "even talking" of Britain leaving the European Union is "anti-jobs and anti-business" as it would create "huge uncertainty" for British businesses.
The shadow chancellor told business leaders that David Cameron's "arbitrary" plan to hold an in-out referendum by 2017 on Britain's EU membership "puts Tory party politics before the national economic interest".
By contrast, he said that Labour would "hold an in-out referendum if there was a further transfer of powers – a prospect we believe is possible, but unlikely in the next parliament.."
"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," he told the British Chambers of Commerce's annual conference in London.
Balls also called on firms to work with Labour to improve UK living standards and safeguard the economic recovery.
He said: "At a time when politicians and business leaders often seem to compete with each other for bad headlines, be it MPs expenses, tax avoidance schemes or rising energy prices, none of us can afford to bury our heads in the sand and ignore the legitimate and mainstream concerns of people across our country that our economy is not currently working for them and their families.
"That is why we believe it is so vital that government works closely with all businesses - large and small: to promote open markets, competition and long-term wealth creation; and to reform our economy so that, by using and investing in the talents of all, we can deliver rising living standards, not just for a few but for everyone in every part of the country."
Previous speakers to the British Chambers of Commerce conference included education secretary Michael Gove, who called for an end to the "preventable blights" of literacy and innumeracy "within our lifetime".
Apprentice star and Tory small business ambassador Karren Brady used her speech to praise the government's record, saying that: "I truly believe that this is the best country to start and grow a small business".