A first voter survey suggests that a majority of British voters have opted to remain in the European Union.
Fifty-two per cent voted for staying in the bloc, while 48 per cent voted for leaving, polling company YouGov says after polling stations close, adding that its data are too close to call.
The historic referendum had raised fears that a vote for Brexit could stoke anti-EU sentiments and lead to similar referendums across the EU.
YouGov asked some 5,000 voters how they cast their ballots, but the survey does not conform to the high standards of an exit poll.
A similar survey after the Scottish referendum yielded a reliable result in 2014, while the method turned out to be unreliable in British parliamentary elections last year.
The Electoral Commission expects to declare the final outcome Friday morning, but a trend may emerge already during the night as local results are made public.