Nigel Farage (center), the leader of the UK Independence Party, joins supporters in celebrating the results of the EU referendumoutside the Houses of Parliament in London on Friday. TOBY MELVILLE / REUTERS

Scotland voted overwhelmingly to remain in the European Union, and Nicola Sturgeon, the country's first minister, hinted that her Scottish National Party may seek a fresh referendum on Scottish independence. In 2014, the SNP narrowly lost a national vote on quitting the United Kingdom.

Cameron, in an emotional statement outside the prime minister's residence at No 10 Downing Street, said: "The British people have voted to leave the EU, and their will must be respected. ... The will of the British people is an instruction that must be delivered. 

"I do not think I can be the captain to take the country to its next destination. In my view, I think we should have a new prime minister in place by the start of the Conservative conference in October."

The new leader will have two years in which to invoke the start of the exit process, and there have been suggestions that it might be delayed in order to negotiate the best departure terms.

What is not yet clear is the future of Britain's relations with two of its major trading partners, China and the United States.

In Beijing, European Chamber of Commerce Vice-President Patrick Horgan said: "Inevitably, this decision will lead to a period of uncertainty. The priority now must be to move through this period as quickly as possible, and we look to the UK and the EU to clarify how they will manage the process and work together in the future."

The US for years has had British membership in the EU as a core part of its European policy. The UK and China are experiencing what Cameron and President Xi Jinping labeled "a golden era" after the Chinese leader's state visit to Britain in October.

As voting got underway, Foreign Ministry spokeswoman Hua Chun-ying told a regular news conference in Beijing on Friday: "We follow closely the upcoming referendum in the UK. We respect the choice made by the British people. China always supports the integration of Europe, and is happy to see a united, robust and stable EU play a significant role in international affairs."

Chen Yingqun contributed to this story.