Jamie Dimon, chief executive of the banking giant, said up to a quarter of its staff could be forced to leave the UK if negotiators fail to secure a favourable Brexit deal on financial services cross-border trade.
Speaking on the sidelines of the World Economic Forum in Davos, Switzerland, Mr Dimon said: “If we can’t find reciprocal recognition of rules – and there are a lot of people who are mad with the Brits for leaving and want their pound of flesh – then it could be bad.
“It could be more than 4,000.”
The banking chief later lowered this figure saying that JP Morgan could expect to move between 500 and 1,000 staff to cities including Frankfurt, Dublin and Luxembourg.
He was among a number of financial heavyweights raising concerns about Brexit.
Jes Staley, CEO of Barclays, was cautious but remained positive.
He said: “The Bank of England has done quite a good job of navigating thus far post-Brexit, but there is a new competitive environment out there and we’ve got Brexit coming up, so the UK has got challenges.
“It’s gone from the fastest-growing G7 economy to the slowest-growing, but we believe in the UK, and believe in the ultimate prospects of Great Britain.”
Mr Staley and other financial leaders met Theresa May and Chancellor Philip Hammond this month to discuss the impact of Brexit.
The business chiefs said it was important for there to be as much certainty as possible as foreign banks are expected to begin moving certain operations and staff to the EU if negotiators fail to agree to a Brexit transition period.
Mr Staley said: “It’s complicated, I think, the Brexit negotiations, but our dialogue with the Bank of England, with the regulators, with Her Majesty’s Treasury, has been very constructive, and we think there’s a keen interest by the British Government to keep London competitive, and to keep Barclays competitive.
“What I think is important is that we are listened to, and that we have a voice in the discussions with the Chancellor and with the Prime Minister, and we have that, so let’s see what happens over the next year.”
The Goldman Sachs boss Lloyd Blankfein has been open with his Brexit concerns and support for Frankfurt as an alternative location. The bank has already taken out ten floors in a building in Frankfurt in preparation for March 2019.
According to new research by the think tank ‘The UK in a Changing Europe’, the financial services sector is one of the least vulnerable to Brexit as it is already highly globalised and not dependent on th EU’s markets.