Update 10.10am
UK financial markets braced for one of their most volatile days ever today, with sterling hitting a 31-year low in its biggest fall in history after Britons voted to leave the EU and David Cameron said he would resign as prime minister.
Government bond yields pointed to a new record low and UK and European stocks opened 8 percent lower, reflecting widespread alarm in the financial community over the uncertainty and volatility likely to be unleashed by the Brexit vote.
The pound had hit a 2016 high above $1.50 shortly after an earlier opinion poll showed an outcome in favour of 'Remain', but fell nearly 17 cents from that peak as it became clear that the 'Leave' camp had won the landmark referendum.
The British currency's fall of almost 10 per cent was historic, marking a decline greater than anything seen since free-floating system of exchange rates was introduced in the early 1970s.
It was even bigger than on 'Black Wednesday' in 1992, when billionaire financier George Soros was instrumental in pushing the pound out of the Exchange Rate Mechanism.
London bankers working through the night said they hadn't seen anything like the volatility sweeping across UK assets.
"The word 'unprecedented' is often used too much, and people often reach for the hyperbole. But this is truly unprecedented," said Steven Major, head of global rates strategy at HSBC in London.
HSBC cut its forecast for the pound to $1.20 by the end of this year, and several other banks said they expect the value of the British currency to fall further too.
Sterling fell as low as $1.3305, its weakest level against the dollar since September 1985, before recovering some ground to $1.3750. It fell as much as 6 percent against the euro and 15 percent against the yen.
The cost of insuring against swings in the sterling/dollar exchange rate jumped to 53.375 percent, the highest since at least 1998, before easing back.
Banks are likely to be in the spotlight after the Hong Kong listings of HSBC and Standard Chartered plunged as much as 10 percent in Asian hours. Shares in UK banks Barclays and Royal Bank of Scotland were down 30 percent.
Bond trading platform Tradeweb opened earlier than usual on Friday, and initial data showed a jump in benchmark 10-year UK government bond yields to 1.57 percent from around 1.38 percent late Thursday. But that soon reversed, with Citi and Goldman Sachs among those predicting a fall below 1 percent.
The biggest swings, however, were in the foreign exchange market, where trading went on through the night - albeit in light volumes for much of that time - and sterling tumbled to its lowest against the dollar since September 1985.
"I'm one of the people who was here the last time we were trading at $1.35. It's back to the future, we're back to where we were in 1985," said Nick Parsons, co-head of global currency strategy at NAB.
"We've had a 10 percent decline in six hours. That's simply extraordinary. And a vote to leave provides an existential crisis for Europe," he said.
The referendum's market impact was global. U.S. Treasury yields fell sharply as investors rushed to the safety of U.S. government debt, while other safe-haven assets like the Japanese yen jumped and gold had its biggest rally since the 2008 crisis.
All the major international and British banks in London, including Citi <C.N> Deutsche Bank, JPMorgan, Goldman Sachs and Barclays had traders either working through the night or on call.
On Citi's foreign exchange desk in London, dealers were only accepting voice orders and only desk heads had the authority to approve trades, according to a source at the bank.
Banks had warned clients about volatile trading conditions around the results which may lead to large gaps in prices. Barclays stopped accepting new "stop loss" orders as of 0600 GMT, an extremely rare move for one of the big six banks that dominate the world's biggest financial market.
Investors are now bracing themselves for possible intervention from central banks or finance ministries to restore stability and confidence in markets. So far, there has been nothing concrete, market sources said.
"I'm expecting central bank action, and the Bank of England will probably come in with liquidity provisions. A rate cut will likely come at some point too - not today, that will be panic," said HSBC's Major.
The BoE said in a statement it is "monitoring developments closely", having undertaken "extensive contingency planning and is working closely with Her Majesty's Treasury, other domestic authorities and overseas central banks."