Eight out of 10 businesses back Britain staying in the EU in a new poll.
Only 5% of firms polled for the Confederation of British Industry (CBI) said it would be in their interest for the UK to go alone.
Nearly one in seven companies – 15% – are undecided.
CBI president Paul Drechsler said: “The business view on this issue is absolutely clear.”
But the survey, carried out by pollsters ComRes, involved just 773 of the CBI’s 190,000 members – just 0.4%.
Big businesses accounted for 54% of those in the research, with 38% small and medium sized firms, and the rest from trade associations.
CBI Paul Drechsler insists the poll is ‘comprehensive’
The 773 also equates to a mere 0.02% of the 3.6 million active companies in the UK.
Of those polled, only 34 were in Scotland, 50 in Wales and 80 in Northern Ireland.
Mr Drechsler insisted: “By any standard, 773 polled is a large number.
“This is probably the most comprehensive review of business views in the UK both through our consultation process and our poll.”
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A survey by rival business group the British Chambers of Commerce (BCC) earlier this year questioned 2,133 companies.
It showed nearly 60% of firms wanted to remain in the EU, with 30% wanting to leave.
BCC director general John Longworth recently quit after expressing pro- Brexit views.
The CBI has been criticised in the past for being too pro-EU.
John Longworth quit as BCC director general
Mr Drechsler said: “I am entirely confident that in that poll we have covered a representative group of our members.”
The CBI has also been slammed for supporting certain issues in the past, such as the UK joining the euro.
“The CBI has advocated many things and got the vast majority right,” said Mr Drechsler. “This is not about the past, this is about the future.”
He predicted shop prices will rise if Britain quits the EU.
“As a result of being in the EU, prices are lower than they would be otherwise,” he said.
“In countries where there are tariffs, prices are higher.
“We just don’t know but if you look at the evidence today it would be difficult not to expect so.”
He also said leaving the EU would have a “big impact” on the UK economy in the short term.
“In the longer term we just don’t know,” he added. “We will have a prolonged period of uncertainty and that is bad for business.”
The CBI, which represents firms employing nearly seven million people, says it will not align itself with any EU referendum campaign group.
The ComRes poll indicated that 71% of small and medium-sized firms believed that remaining part of the 28-member bloc would be in the best interests of their businesses, with 11% favouring a Brexit.
Some 85% of CBI members based in London said a Remain result would be in the best interest of their organisation, a figure that dropped to 75% in both the Midlands and East of England.
Will Straw, executive director of the Britain Stronger In Europe campaign, said: “This demonstrates that there is significant support among Britain’s small, medium and large businesses for remaining in Europe.
“Being part of the EU’s free trade single market of 500 million people encourages investment, helps businesses to grow, creates jobs, and keeps prices low. Employers know that leaving would put all of this at risk.”