Brexit - British Prime Minister David Cameron has said he will resign, following the UK's historic vote to leave the European Union.
In the referendum, which followed 43 years of membership, 'leave' won by 52 percent to 48 percent.
Look back at how the historic result unfolded
More on the referendum result
England and Wales voted strongly for Brexit, while London, Scotland and Northern Ireland backed staying in the 28-member bloc.
United Kingdom Independence Party (UKIP) leader Nigel Farage hailed it as the UK's "independence day" but the 'remain' camp called it a "catastrophe".
Mr Cameron, who opposed the exit, said he would resign as prime minister by October.
"I do not think it would be right for me to be the captain that steers our country to its next destination," he told reporters outside his office on Downing St in central London.
Mr Cameron, who choked back tears, gave no detailed timetable but said there should be a new leader by the time his Conservative Party held its annual conference in October.
British Prime Minister David Cameron speaks to reporters in front of 10 Downing St in central London. Photo: AFP
"This is not a decision I've taken lightly but I do believe it is in the national interest to have a period of stability and then the new leadership required," he said.
"I think it's right that this new prime minister takes the decision about when to trigger Article 50 and start the process of leaving the EU."
Bookmaker Ladbrokes had former London mayor and leading Brexit campaigner Boris Johnson as the favourite to succeed Mr Cameron.
The pound fell to its lowest level against the dollar since 1985 as the markets reacted to the vote results.
The referendum turnout was 71.8 percent - with more than 30 million people voting - the highest turnout at a UK election since 1992.
UK MP and former London mayor Boris Johnson gives a thumbs-up after casting his vote. Photo: AFP
UK broadcaster ITV reported that, with 90 percent of votes counted, there were about 1.1 million more votes to leave than to stay.
A final official result was yet to be announced.
What will Brexit mean for the UK - and the EU?
The EU would emerge economically and politically weakened, facing the departure of its most free-market oriented proponent - and one that also wielded a UN Security Council veto and ran a powerful army, Reuters reported.
In one go, the EU was expected to lose about a sixth of its total economic output.
The British pound dropped sharply as results started to show a stronger than expected vote to leave.
It collapsed 15 US cents, easily the biggest fall in living memory and its lowest since 1985.
The euro in turn slid 3.4 percent to $US1.0997.
Quitting the EU could cost Britain access to the EU's trade-barrier-free single market, and was likely to mean it would need to seek new trade accords with countries around the world.
US President Barack Obama said the UK would be at the "back of a queue" for a US pact.
The Financial Times reported ratings agency Standard & Poor's as saying Britain's top-notch "AAA" credit rating was no longer tenable.
A divided Britain reacts
The former leader of the UK's opposition Labour Party, Ed Miliband, had said the predicted result reflected the divisions in the country.
"I really hope [Prime Minister David Cameron] and others cotton-on to the sense of anger and division in the country, because I think that is a massively important part of the votes that we're seeing tonight."
Mr Farage had said Mr Cameron should resign immediately if the result came in for 'leave'.
"We've done it," he said, in a video posted on Twitter. "It's independence day. The ordinary decent people in this country have overturned the big banks, the big businesses."
UKIP leader Nigel Farage reacts as results indicate the UK will leave the EU. Photo: AFP
Mr Cameron had urged Britons to vote 'remain', warning that the alternative was a leap in the dark that would hurt trade and investment, bring about a self-inflicted recession, undermine the pound and push up shopping bills and the cost of holidays.
An aide working in Mr Cameron's office told reporters: "We're in uncharted territory... Everyone's just really tired. They haven't slept."
The chief of the campaign to leave, Matthew Elliott, had earlier said there was little appetite among conservative MPs for Mr Cameron to leave his post, calling on him to instead implement the will of the British people.
Mr Elliott said that, for now, Britain was still in the EU and would be for many months and possibly years to come.
Every Scottish electorate voted to stay, as did Northern Ireland by 55.8 percent.
Scotland First Minister Nicola Sturgeon said the results made it clear the people of Scotland saw their future as part of the European Union.
Republican Irish party Sinn Fein's chairman Declan Kearney, meanwhile, said Britain had forfeited any mandate to represent Northern Ireland.
"This outcome tonight dramatically changes the political landscape here in the north of Ireland and we will be intensifying our case for the calling of a border poll," he said.
Scotland first minister Nicola Sturgeon after placing her vote this morning. Photo: AFP
International reaction
European leaders warned Britain's decision would trigger a wider reflection on the future of the European Union.
European Council president Donald Tusk said the bloc's 27 remaining members were determined to keep their unity.
EU laws would continue to apply until Britain formally left, he said.
German Vice-Chancellor Sigmar Gabriel expressed his frustration online. "Damn! A bad day for Europe," tweeted Mr Gabriel, who is also Germany's Economy Minister.
But anti-establishment and far-right parties in Europe, such as Marine Le Pen's National Front in France and the Alternative For Germany Party, were celebrating the outcome.
Dutch anti-immigrant leader Geert Wilders said his country should also now hold a referendum on its membership of the EU.
The immediate effects of Britain's withdrawal from the EU on New Zealand were likely to be limited, Prime Minister John Key said.
New Zealand would seek a fresh trade deal with the UK in the longer term, he said.
Australian Prime Minister Malcolm Turnbull said he expected a period of uncertainty and some instability in global markets.
"The impact on Australia immediately, directly, from a legal point of view, will be very limited because it will take some years for the United Kingdom to leave the European Union, to negotiate an exit," he told reporters.
- RNZ / Reuters / BBC
'Remain' supporters react as the results of the referendum are announced. Photo: AFP
UKIP MEP Paul Nuttall, right, and Conservative MP Nigel Evans celebrate the likely victory of 'leave'. Photo: AFP
