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, says there is a 50-50 chance 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.
Mr Walker 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 PM to commit to a referendum sooner rather than later. By 2017 this government will have implemented 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."