A trader works at his desk as votes are counted for the EU referendum, in London, Britain June 24, 2016. (Reuters)
Sterling plunged to its lowest in three decades and the value of London's big banks sank by the most since the 2008 financial crisis on Friday as Britain's shock vote to leave the European Union sparked turmoil on global financial markets.
The Bank of England's promise to provide 250 billion pounds of support and extra supplies of foreign currency if needed steadied the ship somewhat, helping halve initial losses for the FTSE 100 and the pound.
But Scottish First Minister Nicola Sturgeon gave markets another knock by saying a new referendum on breaking up the United Kingdom itself was now highly likely, leaving sterling down 8 percent against the dollar on the day.
That was its biggest fall since the system of free-floating exchange rates was introduced in the early 1970s, dwarfing falls on "Black Wednesday" in 1992 when the pound was driven out of the pre-euro Exchange Rate Mechanism.
The moves reflected widespread alarm in the financial community over the uncertainty unleashed by the Brexit vote and London bankers who had worked through the night said it was the most volatile day's trading they had ever seen.
"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 and 92 pence per euro by the end of this year, and several other banks said they expected the value of the British currency to fall further.
Money markets moved fully to price in a cut in official interest rates by December, anticipating the Bank of England will need to take steps to prop up an economy that has already slowed in the run in to Thursday's vote.
Traders said there had been strong buyers of sterling in Asia, however -- possibly including foreign central bank reserve managers stocking up on the pound while it was at its cheapest in decades.
Sterling stood at $1.3680 by midday in London, up from as low as $1.3228 GBP=, its weakest level mid-1985.
"The pound fell a long way very quickly and the talk was that the speculative guys sold it on the first results last night and then bought it back," said Richard Benson, co-head of portfolio management at currency fund Millennium Global.
"At around $1.35 it looks to me to be over and done."
The cost of insuring against swings in the sterling/dollar exchange rate jumped to 53.375 percent GBPSWO, the highest since at least 1998, before easing back.
On stock markets, shares of Britain's big banks took the biggest hit, with Barclays (BARC.L) and Royal Bank of Scotland (RBS.L) falling by 17-19 percent.
Ten-year UK government bond yields dropped to 1.06 percent from around 1.38 percent late Thursday GB10YT=TWEB and Citi and Goldman Sachs both predicted a fall below 1 percent as investors took cover in the perceived security of government debt.
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 since before the signing of the Plaza Accord in 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."
All the major international and British banks in London, including Citi (C.N) Deutsche Bank (DBKGn.DE), JPMorgan (JPM.N), Goldman Sachs (GS.N) and Barclays (BARC.L) 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 on Thursday, an extremely rare move for one of the big six banks that dominate the world's biggest financial market.