Sterling and stock markets around the world fell as Britain voted to leave the European Union in what one broker described as 'a dark day for Britain'.
Financial websites indicate the FTSE 100 will open around 8% lower at around 5,820 when it starts trading at 8am.
Stocks in Europe could do worse – early futures indicate shares in France and Germany could fall by double digits.
Sterling is down 13 cents against the dollar overnight and now buys $1.36.
Futures trading was briefly suspended on the S&P 500 and NASDAQ in the US because they opened down more than 5%.
‘Today we stepped off the economic and political precipice,’ says Richard Champion, deputy chief investment officer at Canaccord Genuity Wealth Management.
‘We now need to work out how we extricate ourselves from the EU, while at the same time hanging on to our national integrity.
‘Sterling, company profits, unemployment, economic growth – a big fat question mark hangs over everything.
‘And we still have to navigate a path through austerity, weak global growth, geopolitical dangers and addressing the UK's structural weaknesses: low productivity growth, poor education, and unsustainable debt.
‘It was hard enough doing this in the EU. Now we all have to roll up our sleeves and make our new status work.'
Large market swings
Overnight trading also saw significant moves as the result of Britain’s result became clear.
Stocks in Japan fell 8.1%, markets in Hong Kong fell 4.7%, India’s Sensex declined 3.5% and shares on mainland China fell around 1.0%.
Britain’s Confederation of Business and Industry said the vote was ‘a momentous turning point in our history’ and called for action to stabilise markets.
‘The country has spoken and it’s for us all to listen,’ said Carolyn Fairbarn, the CBI’s director general.
‘Many businesses will be concerned and need time to assess the implications. But they are used to dealing with challenge and change and we should be confident they will adapt.
‘The urgent priority now is to reassure the markets. We need strong and calm leadership from the Government, working with the Bank of England, to shore up confidence and stability in the economy.
‘The choices we make over the coming months will affect generations to come. This is not a time for rushed decisions.’