The campaign for the UK to leave the European Union has 15.705 million votes, compared with 14.584 million votes for those that want to remain in the bloc, broadcaster ITV says.
ITV says more than 90 per cent of votes have now been counted.
Former London mayor Boris Johnson is understood to be waiting for Prime Minister David Cameron to comment on the vote before he appears publicly to speak about the victory for his Leave campaign.
Attention on Twitter is turning to whether Cameron should stay on as prime minister.
Conservative James Heappey tweeted: "Extraordinary night with an even more extraordinary day to come. Market instability worrying but result is final. PM must stay."
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But Labour MP Chris Evans wrote: "The PM must resign now. He doesn't have the authority or credibility to steady the ship and negotiate for us on the world stage."
