The UK has voted by 52% to 48% to leave the European Union after 43 years in an historic referendum, a BBC forecast suggests.
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.
Voters in Wales and the English shires have backed Brexit in large numbers.
Referendum turnout was higher than at last year’s general election.
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 showing a strong result for Leave in Sunderland and fell as much as 6.5% against the euro.
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.
Labour former Europe Minister Keith Vaz told the BBC the British people had voted with their “emotions” and rejected the advice of experts who had warned about economic impacting of leaving the EU.
He added: “It will be catastrophic for our country, for the rest of Europe and for the rest of the world.”
