Workers begin counting ballots after polling stations closed in the Referendum on the European Union in Glasgow, Scotland, Britain, June 23, 2016. [Photo/Agencies]

Financial markets had initially been reassured by opinion polls showing a likely win for 'Remain', and by comments from prominent anti-EU campaigners that they expected to lose.

Nigel Farage, head of the UK Independence Party, told Sky News as voting ended: "It's been an extraordinary referendum campaign, turnout looks to be exceptionally high and looks like Remain will edge it."

Farage said his prediction was based on "what I know from some of my friends in the financial markets who have done some big polling". Government minister Theresa Villiers, who also campaigned for Britain to leave, told Sky her instinct was that the Remain side had won.

Opinion polls put Remain in the lead by margins of 52-48 or 54-46 percent. The surveys, and Farage's comments, pushed the pound to its highest level this year, above $1.50, but it then plunged on the early results.

Professor Vernon Bogdanor, a politics expert who tutored Cameron at Oxford University, told BBC television there was a possibility that Leave would win.

"One can't deny it's a real kick to the British establishment because all three party leaders have favoured a Remain vote, business on the whole favoured a Remain vote, the financial leaders have favoured a Remain vote. The people have not taken their advice," he said.