Talk of Britain breaking its 40-year ties with the EU gathered pace in January when Prime Minister David Cameron said he would negotiate a new role in Europe and hold a referendum by 2017 asking voters whether they wanted to stay in or leave.
Of 415 firms surveyed by the research company YouGov, 78% thought staying in the EU would be in their best interest. 10% thought it would be better to leave and 12% were not sure.
A separate poll on Tuesday (10 September) found that 43% of Britons would choose to leave the EU and 39% would opt to remain in if a referendum were held now. That was in line with other recent surveys that give the "out" camp a narrow lead.
In the YouGov poll for the Confederation of British Industry (CBI), the main employers' grouping, two-thirds of British companies thought leaving the EU would damage trade with European markets.
47% said it would make it harder to find skilled workers, while 3% said it would be easier. Just over half said it would harm competitiveness, against 15% who thought it would help.
The poll's findings will be questioned by Eurosceptics who think Britain's economy could thrive outside the EU, in part by negotiating new trade deals and by being more flexible.
The survey was conducted across Britain between June 13 and July 30 and sampled a range of companies of different sizes and in different sectors.