Fears are mounting that five of the 15 largest firms in the FTSE 100 could leave amid talk of defections to rival stock exchanges and takeovers by foreign predators.
Oil major Shell and miner Glencore are looking at shifting their main listing from London to New York, while Rio Tinto is under pressure from an activist investor to quit for Sydney.
Susannah Streeter, head of money and markets at Hargreaves Lansdown, said: 'Given the uncertainty surrounding trade, and the unpredictability of Trump's policies, a knock to sentiment may well see companies re-evaluate the relative stability of the London market.
'There can be no doubt that New York has most of the momentum right now, but with equity market valuations there looking stretched by historic standards, there remains the risk that any switch to the US could come at a deceptively inopportune moment.'.
Gambling industry insiders think Ladbrokes and Coral owner Entain, whose 50-50 joint venture BetMGM is making waves in the US, could go to New York.
