The UK has voted by 52% to 48% to leave the European Union after 43 years in an historic referendum.
London and Scotland voted strongly to stay in the EU but the remain vote has been undermined by poor results in the north of England.
UKIP leader Nigel Farage hailed the result as the UK’s “independence day”.
The pound fell to its lowest level against the dollar since 1985 as the markets reacted to the results.
The referendum turnout was 71.8% – with more than 30 million people voting – the highest turnout at a UK election since 1992.
Wales and the majority of England outside London voted in large numbers for Brexit.
Labour’s Shadow chancellor John McDonnell said the Bank of England may have to intervene to shore up the pound, which lost 3% within moments of the first result.
A Labour source said: “If we vote to leave, Cameron should seriously consider his position.”
But pro-Leave Conservatives including Boris Johnson and Michael Gove have signed a letter to Mr Cameron urging him to stay on whatever the result.