Bank of England governor Mark Carney has said volatility "can be expected" in the wake of the Brexit vote but said the Bank is prepared to provide £250bn to support markets.
He said: "Some market and economic volatility can be expected as this process unfolds.
"As a backstop, and to support the functioning of markets, the Bank of England stands ready to provide more than £250bn of additional funds through its normal facilities."
"The Bank will not hesitate to take additional measures as required as markets adjust and the UK economy moves forward."
As he spoke, more than £100bn was being wiped from the FTSE and the pound was crashing against the dollar.
Mr Carney also offered reassurance that there will be no immediate changes as a result of the vote.
He said: "There will be no initial change in the way our people can travel, in the way our goods can move or the way our services can be sold. And it will take some time for the United Kingdom to establish new relationships with Europe and the rest of the world."