The UK's financial sector would face heightened risk and an exodus of well over 10,000 jobs without certainty over Britain's Brexit deal, MPs have heard.
Xavier Rolet, chief executive of the London Stock Exchange, said his customers "simply would not wait" and would move operations if the LSE did not have a clear path for continuing its global operation.
Speaking to MPs on the Treasury Select Committee, Mr Rolet said there was no doubt over the scale of the economic impact on jobs within the financial services sector.
"I'm not just talking about the clearing jobs themselves which number into the few thousands. But the very large array of ancillary functions, whether it's syndication, trading, treasury management, middle office, back office, risk management, software, which range into far more than just a few thousand or tens of thousands of jobs. They would then start migrating."
His comments came as Douglas Flint, group chairman of HSBC, said the bank would take "pre-emptive action" to move jobs to France, the Netherlands or Ireland before the Article 50 process is complete.
He said the lender's heavyweight clients expected financial industry to adjust to meet their needs post-Brexit, but warned smaller firms were more likely to suffer under the weight of uncertainty.
Banks have issued stark warnings since the Brexit vote, claiming thousands of jobs would shift to rival financial centres across Europe and the United States if Britain loses the right to sell financial services to the EU.
US banking giant JP Morgan said 4,000 jobs would go, Goldman Sachs threatened to move 2,000 roles if Britain loses passporting rights and HSBC claimed it would transfer 1,000 positions from London to Paris following the Brexit vote.
However, the resilience of the UK economy since the EU referendum result has cast doubt over doom-laden economic reports suggesting Britain would suffer a sharp slowdown.
UK court to hear challenge over single market
A legal challenge to the British government over whether the UK's exit from the European Union will automatically take it out of the single market will be heard in court next week, the group that initiated the action has said.
It is one of several legal battles over how Britain should go about quitting the EU and trading with it afterwards – a conundrum requiring a trade-off between Brexit voters' desire for immigration controls and the economic need for good trading terms with the bloc.
The government argues that Britain's exit from the EU will also end its membership of the European Economic Area (EEA), which provides access to the single market and its free movement of goods, capital, services and people.
British Influence, the group behind the challenge, says the government should approach Brexit on the basis that Britain would remain part of the EEA, which includes EU member states as well as Norway, Iceland and Liechtenstein.
A hearing will take place in the High Court in London next week, most likely on 20 January, British Influence said.