Staying in the European Union is "overwhelmingly" in Britain's interest, although reforms are urgently needed, business leaders have said.
The Confederation of British Industry highlighted research showing that EU membership was worth between £62 billion and £78 billion, around 4-5% of the country's total economic output.
The CBI's enthusiasm for the European Union contrasted with fellow business group the Institute of Directors, which warned in September that the EU had "lost its way".
The CBI's announcement comes as it holds its annual conference in London today, with the group pushing for a number of EU reforms including removing barriers to e-commerce, becoming more outward-looking, and re-focusing the work of EU commissioners.
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."
UKIP leader Nigel Farage said: "There is this deeply flawed view that leaving the EU would somehow mean a sudden end to trading with Europe.
"The CBI does not consider the more realistic option that if we left the single market and freed ourselves from its red tape and politicised agenda we would still be able to continue to trade strongly with Europe on our own terms. Leaving the EU would not mean turning our back on Europe."
He went on: "We are far better getting out of the EU and building a fresh relationship with our European neighbours. That is what the Government and organisations like the CBI should be looking at rather than clinging to the vain hope that the EU project can be miraculously cured of its ills."
Liberal Democrat Chief Secretary to the Treasury Danny Alexander said: "This compelling report makes a powerful business case for the UK's continued membership of the EU.
"EU membership is one of the reasons why Britain is an attractive place to invest, as businesses in India told me last week.
"Liberal Democrats know that we are all better off with Britain in Europe.
"We can only build a stronger economy working together with our neighbours, exerting maximum influence in our continent and leading the reform agenda as the most liberal economy in Europe."
Shadow minister for Europe Gareth Thomas said: "The CBI's report clearly shows that the business community doesn't want to see Britain sleepwalk out of the EU.
"Despite this report making clear that British business wants us in Europe, the Tory Party still wants David Cameron to take us out."