The Dow Jones Industrial Average plummeted more than 400 points as global markets descended into chaos following Britain's vote to leave the European Union.
Major banking and financial services firms BNP Paribas and JPMorgan are considering moving their London bases to Paris, according to reports.
ITV News understands the firms are considering the move following the EU referendum result.
The International Monetary Fund (IMF) has urged the UK and EU to ensure a "smooth transition" to a new economic relationship after the Brexit vote.
Christine Lagarde, managing director of the IMF, said it strongly supported the Bank of England and ECB's commitments to "supply liquidity to the banking system and curtail excess financial volatility".
She said: “We urge the authorities in the U.K. and Europe to work collaboratively to ensure a smooth transition to a new economic relationship between the UK and the EU, including by clarifying the procedures and broad objectives that will guide the process.
“We will continue to monitor developments closely and stand ready to support our members as needed.”
British Airways owner IAG has issued a profit warning after British voters chose to leave the European Union.
It came as the company's share price fell 19% to 425.6p.
"Following the outcome of the referendum, and given current market volatility, while IAG continues to expect a significant increase in operating profit this year, it no longer expects to generate an absolute operating profit increase similar to 2015," the company said in a statement.
The firm insisted it believes the vote "will not have a long-term material impact on its business", but added that it experienced "a weaker than expected trading environment" in the run-up to the referendum.
Economists have started to downgrade their forecasts for UK growth, predicting Britain could head into recession unless a quick deal can be done.
IHS Global Insight said it is "substantially cutting" its GDP growth forecasts to 1.5% from 2% for 2016 and to 0.2% from 2.4% for 2017.
Capital Economics downgraded its forecast for 2016 from just above 2% to 1.5%.
Fitch said the vote for Brexit will be "credit negative for most sectors in the UK", triggered by "weaker medium-term growth and investment prospects and uncertainty about future trade arrangements".
The UK's decision to leave the European Union will have "significant implications" for the country, the Financial Conduct Authority (FCA) has said.
The regulator said the long-term impact of the referendum result would depend in part on the relationship the UK forms with the EU in the future.
It said it was in "very close contact" with firms it supervises as well as the Treasury, the Bank of England and other UK authorities.
"Consumers’ rights and protections, including any derived from EU legislation, are unaffected by the result of the referendum and will remain unchanged unless and until the Government changes the applicable legislation," the FCA said.
"The longer term impacts of the decision to leave the EU on the overall regulatory framework for the UK will depend, in part, on the relationship that the UK seeks with the EU in the future.
"We will work closely with the Government as it confirms the arrangements for the UK’s future relationship with the EU."
The European Central Bank has said it is ready to provide euro and foreign currency liquidity to European nations as Brexit sent global financial markets into a tailspin.
Following the outcome of the UK referendum, the European Central Bank is closely monitoring financial markets and is in close contact with other central banks.
The ECB stands ready to provide additional liquidity, if needed, in euro and foreign currencies. The ECB will continue to fulfill its responsibilities to ensure price stability and financial stability in the euro area.
ECB officials added that the Brexit would have a limited impact on the particularly fraught Greek economy.
Mark Carney has said there will be a period of uncertainty and adjustment after the vote to leave the European Union.Read the full story ›
The FTSE-100 has fallen by more than eight percent within minutes as markets opened on Friday following Britain's decision to leave the European Union.
Bank stocks have fallen by 30%, ITV News Business Editor Joel Hills reports:
Here they come. Biggest fallers banks (Barclays -30% RBS -35% Lloyds -29%) and construction companies (Redrow -76%) https://t.co/L3DkmTnDeP