DAVID Cameron has announced he will quit as Prime Minister by October after a humiliating defeat in the referendum which ended with a vote for Britain to leave the European Union.

Mr Cameron announced his decision outside 10 Downing Street minutes after the markets opened with carnage in the City of London.

More than £100 billion was wiped off the FTSE 100 as the index fell more than 7%, while the pound also crashed 8% against the US dollar.

Flanked by wife Samantha, Mr Cameron said he had informed the Queen of his decision to remain in place for the short term, but hand over to a new Prime Minister by the time of the Conservative annual conference.

"The British people have voted to leave the European Union and their will must be respected," said Mr Cameron. "The will of the British people is an instruction that must be delivered."

His voice breaking, Mr Cameron said: "I love this country and I feel honoured to have served it and I will do everything I can in future to help this great country succeed."