Business wants an early referendum on the UK's membership of the European Union amid concerns that a delay would encourage a chance to "whack the political elite", according to an industry leader.
Simon Walker, director general of the Institute of Directors, believes there is a 50-50 chance that British voters will opt to leave the EU.
He will tell the annual convention of the IoD that hardly any member of the organisation wanted to leave the EU as a matter of principle, but most have doubts about Europe's institutions and wanted to see reform.
He will tell an audience of 2,000 businessmen and women: "The British political class, always prey to groupthink, has had two shocks this year, with the decisive re-election of a Conservative government and the choice of Jeremy Corbyn as leader of the Labour Party.
"I believe a third shock, Britain voting to leave the European Union, is at least a 50-50 possibility. Polling puts the issue on a knife-edge."
Mr Walker will say his concern is the timing of the referendum, adding: "I hope this government will not drag out the referendum process any longer than necessary. While our members believe some uncertainty is a price worth paying to resolve EU membership, delay puts a brake on decision-making, investment and the vigour of their businesses.
"A referendum in two years' time will see British concerns caught up in the crossfire of the French elections, in May, and the German elections, in September.
"And there are domestic factors that should encourage the Prime Minister to commit to a referendum sooner rather than later. By 2017 this government will have implemented spending cuts that, while necessary, will not be popular. The third year of an election cycle is a difficult time for any administration.
"There is a real possibility that a 2017 referendum would be a short-term judgment on the Government: a chance to whack the political elite.
"If, after deliberation, the public votes to leave Europe, our members will have to accept it, and the period of uncertainty for business that will follow. They will be less philosophical if carelessness and domestic discontent led to an accidental Brexit."
Mr Walker will say that people should be focusing on the main issue of whether the UK would be better or worse off leaving the EU.
Lord Mandelson and Lord Lawson will debate EU membership, while speakers at the convention, being held at London's Royal Albert Hall, include NHS chief executive Simon Stevens and International Rescue Committee chief executive David Miliband.