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.

The Prime Minister 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."

Mr Cameron said he accepted the decision of the electorate, which voted by 52% to 48% to quit the EU.

He said he would leave it to his successor to invoke Article 50 of the Lisbon Treaty, which kicks off the two-year process of negotiating a new trade relationship with the UK's former partners.

"The country requires fresh leadership to take it in this direction," said Mr Cameron. "I will do everything I can as Prime Minister to steady the ship over the coming weeks and months, but I don't think it would be right for me to try to be the captain that steers our country to its next destination."

His announcement will trigger a battle for the Conservative leadership - and the keys to Number 10 - likely to feature Brexit standard-bearer Boris Johnson taking on figures such as Home Secretary Theresa May, who took a low profile in the referendum campaign.