The Confederation of British Industry (CBI) said a survey of its members showed 80 per cent believed remaining in the EU would be "in the best interests of their business and the wider UK economy".
Just five per cent said that "Brexit" -- or Britain exiting the EU -- would be in their best interests, while 15 per cent were uncertain.
The CBI survey, carried out by independent pollsters ComRes, quizzed 773 small, medium and large companies across Britain.
Prime Minister David Cameron is leading the battle to keep Britain in, but several key members of his Conservative party -- notably Mayor of London Boris Johnson -- have joined the Leave campaign.
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However, the CBI, which represents companies employing almost seven million people in Britain, stressed today that it would not back any EU referendum campaign group.
The ComRes survey formed part of the CBI's consultation process, which prompted its highest policy-making committee to recommend making the economic case for staying in the EU.
"Walking away makes little economic sense and risks throwing away the many benefits we gain from being part of the EU.
"Our members tell us that having guaranteed access to a tariff-free market of 500 million people, and to more than 30 global trade deals covering 50 countries, are significant advantages that outweigh the frustrations.
"A minority of members want to leave the EU. We will continue to respect and reflect their views and campaign for EU reform to get a better deal for all businesses.