Prime Minister David Cameron must hold a planned referendum on Britain’s European Union membership as soon as possible or risk “accidental Brexit”, the head of the Institute for Directors, a business lobby group, will saylater today.

Cameron has promised to reform Britain’s EU ties, then hold a referendum on whether to remain a member, by the end of 2017. He has said he will hold it earlier if he can, but Foreign Secretary Philip Hammond has warned the schedule is tight for a 2016 referendum.

Polls show the gap between Britons who want to remain in the EU and those who want to leave remains tight.

“Britain voting to leave the European Union is at least a 50-50 possibility,” IoD Director General Simon Walker will tell an audience of 2,000 business leaders in London on Tuesday, according to extracts released in advance.

“My concern is the timing of the referendum. I hope this government will not drag out the referendum process any longer than necessary.”

A 2017 referendum would see Britain’s renegotiations caught up among French and German elections that year, Walker will warn. Domestically, it risks becoming a vote on the government’s planned spending cuts.

“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,” he will say.

“If … 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.'”