Global stock markets plunged on Friday, and sterling saw its biggest one day drop in history after Britons voted by 52-48 percent to exit the EU, which it joined more than 40 years ago.

Ratings agency Moody's downgraded its outlook for Britain, saying its creditworthiness was now at greater risk as the country would face substantial challenges to successfully negotiating its exit from the bloc.

European Commission President Jean-Claude Juncker said he wanted to begin negotiating Britain's departure immediately.

"Britons decided yesterday that they want to leave the European Union, so it doesn't make any sense to wait until October to try to negotiate the terms of their departure," Juncker told Germany's ARD television station.

Prime Minister David Cameron announced on Friday he would resign after leading the failed campaign to stay in the bloc, and said someone else should take the lead in negotiating the unprecedented and complicated extrication.

He suggested his replacement would be in place by October. That person could be his Conservative Party rival Boris Johnson, the former London mayor who became the most recognisable face of the Leave camp and who is now favourite to succeed him.

Britain's decision to leave the EU is the biggest blow since World War Two to the European project of forging greater unity.

The United Kingdom itself could also now break apart, with the nationalist leader of Scotland, where nearly two-thirds of voters wanted to stay in the EU, saying a new referendum on independence from the rest of Britain was "highly likely".

Scottish government ministers were meeting on Saturday to decide their next move. German Chancellor Angela Merkel will meet French, German and Italian leaders in Berlin on Monday to discuss future steps, and the foreign ministers of Germany, France, Italy, Belgium, the Netherlands and Luxembourg, will meet on Saturday morning.    

U.S. President Barack Obama on Friday tried to limit the fallout from Britain's vote to leave the European Union which threatens to harm the US economic recovery and distract US allies from global security issues.

Obama vowed that Washington would still maintain both its "special relationship" with London and close ties to Brussels, but stood by his warning that Britain would move to the back of the queue when it came to trade deals.

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