Sterling plunged to a 30-year low and markets fell on Friday after Britain opted to leave the European Union, spurring calls for similar votes in other nations in an already troubled EU.
With counts declared in all 382 voting areas, Leave won 52 per cent of the vote and Remain won 48 per cent, marking the first time that a country has decided to leave the EU.
The vote comes at a time when the union is also dealing with internal tensions about its migration crisis, as well as with high unemployment and growing anti-EU sentiments among the European public.
The mostly Conservative Vote Leave platform and the right-wing UK Independence Party had lobbied for a Brexit, arguing that this was the only way to protect Britain's sovereignty and control EU migration.
"Let June 23 go down in our history as our independence day," UKIP leader Nigel Farage said.
The biggest effect from the British referendum was that other countries were starting to doubt the EU, Farage said. Indeed, other eurosceptic parties across Europe heralded the Brexit vote as an argument for holding their own anti-EU referendums.
In another sign of potential trouble ahead, some observers warned that the decision to leave the EU would encourage support for a second referendum on independence for Scotland, which voted for Remain by a large majority.
"The United Kingdom has made a decision against the interests of the Scottish people and that will have consequences," SNP lawmaker Fiona Hyslop told the BBC.
Irish nationalist party Sinn Fein said the British government had "forfeited any mandate" to represent the interests of Northern Ireland, where a majority also voted to remain.
The Remain camp, led by Prime Minister David Cameron, had argued that an exit from the EU's common market would hurt the British economy and cost jobs, a view that is shared by other EU countries, as well as by leading economists around the world.
In addition, economists have warned that financial markets could be thrown into turmoil if Britons vote the EU's second-largest economy, after Germany, out of the bloc.
Early on Friday, the pound fell below the 1.35-dollar level for the first time since 1985. It remained to be seen what would happen once European markets open.
Most cities in north-eastern England returned large majorities for Leave following Thursday's vote, while Scotland and Northern Ireland overwhelmingly voted for Remain.
Several areas of London also voted strongly for Remain, with 79 per cent opting to stay in the EU in the city's south-eastern borough of Lambeth, as national turnout averaged 72 per cent.
In contrast, the vote for Leave was 76 per cent in Boston in eastern England, a market town with the country's highest proportion of eastern European migrants, according to a national census in 2011.
About 46.5 million people were registered to vote.
A YouGov survey of nearly 5,000 people who voted showed that, among voters with a standard school education, 66 per cent voted for a Brexit, while 71 per cent of those with university degrees voted in favour of remaining in the EU.
Prime Minister David Cameron, under pressure from those in his own Conservative party and a rising euroscepticism among British voters, had promised in 2013 to hold a referendum on EU membership.
He called the vote in February after negotiating a package of reforms with the bloc that he said would Britain a "special status" if voters opt to remain in the EU.