BY GUY FAULCONBRIDGE AND KATE HOLTON for REUTERS -- Britain voted to leave the European Union, forcing the resignation of Prime Minister David Cameron and dealing the biggest blow to the European project of greater unity since World War Two.
Global financial markets plunged on Friday as results from a referendum showed a 52-48 percent victory for the campaign to leave a bloc Britain joined more than 40 years ago.
The pound fell as much as 10 percent against the dollar to touch levels last seen in 1985, on fears the decision could hit investment in the world's fifth-largest economy, threaten London's role as a global financial capital and usher in months of political uncertainty. The euro slid 3 percent.
World stocks saw more than $2 trillion wiped off their value, with indices across Europe heading for their sharpest one-day drops ever. Britain's big banks took a $100 billion battering, with Lloyds (LLOY.L), Barclays (BARC.L) and RBS (RBS.L) plunging as much as 30 percent at one point. [MKTS/GLOB]
The United Kingdom itself could now break apart, with the 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".
An emotional Cameron, who led the "Remain" campaign to defeat, losing the gamble he took when he called the referendum three years ago, said he would leave office by October.
"The British people have made the very clear decision to take a different path and as such I think the country requires fresh leadership to take it in this direction," he said in a televised address outside his residence.
"I do not think it would be right for me to be the captain that steers our country to its next destination," he added, choking back tears before walking back through 10 Downing Street's black door with his arm around his wife Samantha.
Quitting the EU could cost Britain access to the EU's trade barrier-free single market and means it must seek new trade accords with countries around the world. A poll of economists by Reuters predicted Britain was likelier than not to fall into recession in the coming year.
The EU for its part will be economically and politically damaged, facing the departure of a member with its biggest financial center, a U.N. Security Council veto, a powerful army and nuclear weapons. In one go, the bloc will lose around a sixth of its economic output.
"It's an explosive shock. At stake is the break up pure and simple of the union," French Prime Minister Manuel Valls said. "Now is the time to invent another Europe."
The result emboldened eurosceptics in other member states, with French National Front leader Marine Le Pen and Dutch far-right leader Geert Wilders demanding their countries also hold referendums. Le Pen changed her Twitter profile picture to a Union Jack and declared "Victory for freedom!"
The vote will initiate at least two years of divorce proceedings with the EU, the first exit by any member state. Cameron - who has been premier for six years and called the referendum in a bid to head off pressure from domestic eurosceptics - said it would be up to his successor to formally start the exit process.
His Conservative Party rival Boris Johnson, the former London mayor who became the most recognizable face of the Leave camp, is now widely tipped to seek his job.
Johnson left his home to jeers from a crowd in the mainly pro-EU capital. He spoke to reporters at Leave campaign headquarters, taking no questions on his personal ambitions.
"We can find our voice in the world again, a voice that is commensurate with the fifth-biggest economy on Earth," he said.
There was euphoria among Britain's eurosceptic forces, claiming a victory over the political establishment, big business and foreign leaders including U.S. President Barack Obama who had urged Britain to stay in.
"Let June 23 go down in our history as our independence day," said Nigel Farage, leader of the eurosceptic UK Independence Party, describing the EU as "doomed" and "dying".
On the continent, politicians reacted with dismay.
"It looks like a sad day for Europe and Britain," said German foreign minister Frank-Walter Steinmeier. His boss Angela Merkel invited the French and Italian leaders to Berlin to discuss future steps.
The shock hits a European bloc already reeling from a euro zone debt crisis, unprecedented mass migration and confrontation with Russia over Ukraine. Anti-immigrant and anti-EU parties have been surging across the continent, loosening the grip of the establishment that has governed Europe for generations.
U.S. presidential candidate Donald Trump, whose own rise has been fueled by similar anger at the political establishment, called the vote a "great thing". Britons "took back control of their country", he said in Scotland where he was opening a golf resort. He criticized Obama for telling Britons how to vote, and drew a comparison with his own campaign.