Sterling has fallen to a 31-year low - marking a sharper dive even than on Black Wednesday in 1992 - as the Leave campaign closed in on victory in the EU referendum.
Shares in London are also expected to endure a torrid session with futures trading predicting a fall of 8%, or around 500 points, in the FTSE 100 Index - knocking more than £130bn off the value of its constituent companies. There were heavy falls on Asian markets overnight.
The pound saw wild swings after the polls closed - research showed the Remain side marginally ahead at 11pm and the pound headed sharply up to a six-month high against the dollar at $1.50.
But after regional results began to paint a clearer picture, the pound nosedived by more than 10% to $1.33.
It was the lowest level for sterling against the US currency since 1985. Sterling also saw a sharp fall against the euro, down 8%.
Yahoo Finance / Sterling Dollar five day chart
The collapse in the pound will be of immediate concern to holidaymakers who will find their money does not go as far in summer break destinations from California to the Costa del Sol.
It is also likely to push up some shop prices as imported goods will cost more for British consumers.
UK companies will also find that materials they buy, for example in dollars, will be more expensive.
However, the fall in the pound could make some British goods cheaper for foreign buyers, helping exports.
Elsewhere, Japan's Nikkei index tumbled by 7% while in Hong Kong, the Hang Seng shed 5%. HSBC and Standard Chartered - listed in Hong Kong as well as London - fell 9% and 10%.
The Bank of Japan said it was ready to supply liquidity to limit the market impact of a Brexit vote, in an early sign of central banks stepping in to mitigate the turmoil.
The price of gold - a traditional safe-haven during volatility - climbed to a two-year high.
Meanwhile, oil slipped by $3, or 5%, with expectations that the shockwaves from a Brexit will limit global demand.
But it was the fall in sterling that was the stand-out move.
Sky's Economics Editor Ed Conway said: "The pound has absolutely gone through the floor.
"This kind of thing almost never happens, you have to go back to eras like Black Wednesday - when the pound left the exchange rate mechanism - to see such instant falls."
He added: "More than a 10 cent move is, as far as I know, basically unprecedented.
"This is likely to go down in history as the most volatile day of trading for sterling in modern record.
"The pound is seen as a barometer of how well the UK economy is doing, so a lot of people will be very worried indeed."
Speaking before the official result was announced, Joe Rundle, head of trading at ETX Capital, said: "We've never seen anything quite like this.
"Sterling has dived from a high of 1.50 dollars to below 1.36 dollars in a matter of hours.
"These moves are pretty well unheard of but will only get worse if Leave does win, which it is looking increasingly likely now."
The reaction was all the greater for markets since they had confidently been pencilling in a Remain victory on the back of a series of predictions from pollsters and bookmakers - sending the FTSE 100 and Index and the pound higher in Thursday trading.
The billionaire currency investor George Soros warned earlier this week that the pound could go as low as $1.15 in the event of a Leave vote.
