More than £100billion was wiped off the FTSE 100 - the biggest fall in UK history - as Britain's decision to leave the EU caused panic across the globe.
London's premier index plunged 458 points to 5,880 - down 7.19 per cent - as experts warned of more carnage to come.
The collapse is far greater than Britain suffered after the Winter of Discontent in 1978, Black Wednesday in 1992 and when Lehman Brothers shut in 2009.
The UK's banks have been the hardest hit with RBS shares down 34 per cent, Lloyds down 28 per cent and Barclays down 30 per cent.
Shares in Britain's largest building and insurance companies were also devastated.
Brexit has also caused panic on the financial markets today as the pound dropped more than 10 per cent against the dollar and Britain was about to lose its AAA rating.
Bank of England governor Mark Carney said "some market and economic volatility can be expected" in the wake of the Brexit vote, adding that the Bank was well prepared.
Share trading in Japan was even halted by panicked traders as the markets went into freefall when the referendum result became clear.
Traders around the globe were left in despair as their own markets fell because of the Brexit decision. The pound lost more than 10 per cent against the dollar and plunged below $1.35.
Dennis de Jong, managing director of UFX.com, said: "This is simply unprecedented. The pound has fallen off a cliff and the FTSE is now following suit. Britain's EU referendum has been a cloud hanging over the global economy for the past few months and that cloud has got very dark this morning.
"The markets despise uncertainty, yet that is exactly what they're faced with this morning. The shockwaves are likely to reverberate for some time and the warning lights are flashing brighter now than ever."
Britain is set to lose its triple-A credit rating after voting to leave the European Union. S&P, the only ratings agency to maintain a AAA rating, has said that maintaining it is untenable.
Moritz Kraemer, chief ratings officer for S&P, told the Financial Times: "We think that a AAA-rating is untenable under the circumstances."
David Cameron resigned over the result today and moments after Mr Cameron finished speaking, Mr Carney made a televised statement from the Bank of England in Threadneedle Street urging calm.
He said it was "inevitable" there would be a period of "uncertainty" in the wake of the Brexit vote, and admitted it would take "some time" for the UK to forge new arrangements with the EU and the rest of the world.
But the governor - who previously warned that Brexit was the biggest domestic risk to the economy - insisted the Bank and the Treasury had been doing "extensive emergency planning".
"We are well prepared for this," he said.
The markets began to shudder after the first bombshell result in Sunderland, which far exceeded expectations in the Leave camp, provoked fierce new jitters in the City.
The pound immediately plunged three per cent after the Sunderland result and at 2am was more than 5 per cent off against the dollar. The market rebounded some of the lost ground after Wandsworth voted strongly for Remain.
But at around 3.30am as expectations of a Brexit vote grew and it became the bookies favourite for the first time, Sterling dropped like a stone.
The Bank of England has said it will take "all necessary steps" to ensure monetary and financial stability in the wake of the Brexit vote.
In a statement issued following Britain's referendum on the European Union, the Bank said: "The Bank of England is monitoring developments closely.
"It has undertaken extensive contingency planning and is working closely with HM Treasury, other domestic authorities and overseas central banks. The Bank of England will take all necessary steps to meet its responsibilities for monetary and financial stability."
The comments come after economists began to downgrade their forecasts for UK growth, with a recession forecast unless a quick deal with the EU can be done.
Japanese markets reopened after an automatic 10-minute shut down because of the currency drop.
Analysts said the instant dive in the value of the pound was the deepest since Black Wednesday in 1992 - but the figure is expected to rebound over the course of a rollercoaster night.