UPDATED 9.13PM Britain has 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 II.

LISTEN ABOVE: ITV's Alex Hazlehurst speaks with Larry Williams about the result

Global financial markets plunged as results from Thursday's referendum showed a near 52-48 per cent split for leaving.

The pound fell more than 10 per cent against the US dollar to levels last seen in 1985, its biggest one-day fall in history, 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.

European shares plummeted more than 8 per cent on Friday, headed for their biggest ever one-day fall.

Billions of dollars were wiped off European banks' market value, with Britain's Royal Bank of Scotland, Barclays and Lloyds Banking Group among the biggest fallers.

An emotional Cameron, who led the "Remain" campaign but lost the gamble he took when he called the referendum three years ago, said he would resign as prime minister 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 Downing Street office.

"I do not think it would be right for me to be the captain that steers our country to its next destination."

Quitting the EU could cost Britain access to the EU's trade barrier-free single market and mean it must seek new trade accords with countries around the world.

The United Kingdom itself could break apart, with leaders in Scotland - where nearly two-thirds of voters wanted to stay in the EU - calling for a new vote on independence.

The EU for its part will be economically and politically damaged, facing the departure not only of its most free-market proponent but also a member with a UN Security Council veto and powerful army.

In one go, the bloc will lose around a sixth of its economic output. Populist leaders in France and the Netherlands demanded their own referendums to leave.

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, 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 recognisable face of the "Leave" camp, is now widely tipped to seek his job.

The four-month campaign was among the divisive ever waged in Britain, with accusations of lying and scare-mongering on both sides and rows on immigration which critics said at times unleashed overt racism.

It also revealed deeper splits in British society, with the pro-Brexit side drawing support from millions of voters who felt left behind by globalisation and believed they saw no benefits from Britain's ethnic diversity and free-market economy.

A pro-EU member of parliament was stabbed and shot to death in the street a week ago by an attacker who later told a court his name was "Death to traitors, freedom for Britain". Older voters backed Brexit; the young mainly wanted to stay in.

But in the end, concerns over uncontrolled immigration, loss of sovereignty and remote rule from Brussels appear to have trumped almost unanimous warnings of the economic perils of going it alone.

The Bank of England said it would take all necessary steps to secure monetary and financial stability. Global policymakers also prepared for action to stabilise markets, with Japanese Finance Minister Taro Aso promising to "respond as needed" in the currency market.

ECONOMIC IMPLICATIONS OF THE VOTE FOR BRITAIN:

ECONOMIC GROWTH
- Britain's economy would grow more slowly outside the EU than if it stayed in, according to projections by the government, the Bank of England, British research institutes, international organisations and academics.
- Chancellor of the Exchequer George Osborne has warned of a "DIY recession" and the BoE has said a "material slowdown" could result after a Brexit.
- Uncertainty about the future of Prime Minister David Cameron and Osborne, and the possibility of another independence referendum in Scotland, could also weigh on economic growth.
- A small group of pro-Brexit economists has said leaving the EU will boost growth in the years to come, although at least one of them predicts a shallow downturn first.
- The fall in sterling, which on Friday hit its lowest level against the dollar since 1985, could help exporters, although demand in many countries around the world remains weak.
- The OECD says there could be deeper global economic fallout if a Brexit undermines confidence in the EU, a scenario not included in its forecasts.

MONETARY POLICY
- BoE governor Mark Carney has said it is too simple to assume the Bank will cut interest rates from what is already a record low of 0.5 per cent to cushion the economy after a Brexit vote. The BoE says it would have to weigh up slower growth against higher inflation caused by a weakening of the pound.

CURRENT ACCOUNT
- Britain racked up its biggest current account deficit on record in 2015, equivalent to 5.2 per cent of economic output. The shortfall reflected higher flows of dividends and debt payments to foreign investors than similar flows into the country as well as its wide trade deficit. Carney has said a Brexit could test the "kindness of strangers" who fund the balance of payments deficit.

STERLING
- Despite the risk premium already built into sterling, analysts predict the currency will fall further. Before the referendum, investors predicted the pound would hit a multi-decade low about $US1.35.

JOBS
- Most forecasters think Britain's unemployment rate - now at a 10-year low of five per cent - will rise after leaving the EU.
- Wages will probably bear the brunt of any post-Brexit slowdown, the International Monetary Fund says. Britain's National Institute of Economic and Social Research estimates real consumer wages will be between 2.2 per cent and seven per cent lower in real terms by 2030 than if Britain had stayed in the EU.
- By contrast, Economists for Brexit says Britain's labour market could become more dynamic through the repeal of EU regulation and the elimination of some of the EU's highest import tariffs.

TRADE
- Leaders from the US, Japan, Germany and France warned Britain that leaving the EU would hurt its standing as a global trading power.
- President Barack Obama said Britain would join "the back of the queue" for talks with the US, and French President Francois Hollande said leaving the EU would risk Britain's access to the single market.
- Pro-Brexit economists have dismissed the warnings as scaremongering and say Britain could forge trade deals with the EU and countries beyond.