One in five British companies would move at least some of their operations to the European Union in the event of a "no-deal" Brexit, according to a survey on Friday.
A similar proportion said they would cut investment and jobs, the British Chambers of Commerce (BCC) said.
With six months to go until Britain is due to leave the EU, London and Brussels remain deadlocked over the terms of their separation.
"Our evidence is clear - failure to reach a political agreement would have real-world consequences, with significant decreases in both investment and recruitment," BCC Director General Adam Marshall said.
"Government must act urgently and decisively to get a comprehensive deal done."
The BCC survey was conducted before EU leaders last week rejected Britain's proposals for Brexit.
A Reuters poll of economists at the start of this month put the odds of a no-deal Brexit at one in four.
Most economists think Britain's economy would be seriously disrupted by such an outcome. A small minority say Britain would thrive if it defaulted to World Trade Organisation terms for its trade with the EU.
The BCC survey also showed 62 percent of firms taking part had not performed a risk assessment of the impact of Brexit.
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"Firms still don't have answers from government to the most basic questions about future trading conditions," Marshall said
A separate poll conducted by The National Centre for Social Research (NatCen) found that Britons would vote 52 to 48 percent in favour of remaining in the European Union were there to be another Brexit referendum, showing only a small change in public opinion since the 2016 vote.
But researchers cautioned that a narrow victory for those hoping to reverse the decision taken two years ago would be heavily contingent on getting those who did not vote last time to turn out.
The figures are calculated by taking the average of the six most recent surveys by major pollsters between Aug. 21 and Sept. 10.
"True, Remain enjoys a lead in the polls. But that lead remains a narrow one, and there is little sign of it growing," said Senior Research Fellow at NatCen John Curtice.
"It rests quite heavily on the preferences of those who did not vote two years ago, and who by two to one now say they would back Remain. Getting them to turn out to vote in any second ballot might yet prove a considerable challenge."
Campaigners on both sides of the argument have been stepping up their efforts in recent weeks as some Brexiteers argue for a cleaner break from the EU and Remainers say ending membership as currently planned would do economic damage to Britain.