Support among business for Britain staying in the EU has declined since David Cameron announced an in/out referendum three months ago.

Despite warnings about the economic costs of Brexit from the Treasury, the International Monetary Fund and the Organisation for Economic Cooperation and Development, the lead for the remain side has narrowed from 30 points to 17.

The British Chambers of Commerce (BCC) said its survey of 2,200 businessmen and women showed 54% were in favour of staying in the EU, down from 60% when its members were last canvassed in late January and early February.

By contrast, support for leaving the EU rose from 30% to 37% in the period shortly before the prime minister concluded his negotiations on the terms of the country’s membership.

Adam Marshall, acting director general of the BCC, said: “As the EU referendum campaign enters the final straight, the race for the business vote has clearly tightened. Although a clear majority of the businesspeople we surveyed continue to express a preference to remain in the European Union, the gap between remain and leave has narrowed significantly in recent weeks.”

Marshall took over at the BCC after former director general John Longworth resigned after being suspended from his post for voicing support for Brexit.

The findings of the poll will provide comfort to the leave camp, since they suggest that reports predicting the likely economic costs of departure from the EU have not had the desired effect.

In addition, the vast majority of the business leaders questioned said they had decided how they would vote on 23 June. Only 11% said they were prepared to change their minds over the coming weeks.

A breakdown of the BCC data reveals that voting intentions were related to the size of the company and whether it exported. Those representing large firms and those trading with other EU markets expressed the strongest support for “remain”, with the strongest levels of support for “leave” among micro businesses and those running companies servicing the domestic market.

Marshall said: “While only a minority of businesspeople report that the referendum campaign has had a material impact on their firms to date, much larger numbers say they expect significant impacts in the aftermath of the vote.

“Whichever outcome prevails, Westminster must shift its attention back to the economy on 24 June without delay. Growth is softening, and Westminster’s referendum tunnel vision over the past year has meant that far too many key economic issues have been given short shrift or delayed altogether.”

A separate survey from the Recruitment and Employment Confederation found that the uncertainty caused by the EU vote had, along with the introduction of the “national living wage”, caused a shift towards temporary hiring.

With permanent placements growing at their slowest pace since September, the REC chief executive, Kevin Green, said: “Employers are turning to temps and contractors to provide a flexible resource, as a way of hedging any possible change to the UK’s relationship with Europe and the implications this would have on the economy.”