JP Morgan boss Jamie Dimon has said that any British job losses as a result of Brexit would take place over a "period of years" as the US bank prepares for a new regulatory landscape.
The lender employs 16,000 people in the UK, with London hosting its European headquarters, and has previously said that around 4,000 jobs could go if Britain loses the right to sell financial services to the European Union.
Mr Dimon, who backed the Remain campaign, told Financial News: "We've been planning for a range of outcomes because it's still as uncertain as a couple months ago.
"What we know now is that this will be a slow process, and all the staff moves would not happen at once but over a period of years. If there is not a clear transitional period decided early in the process, where passporting rules still apply for a few years after negotiations, then we'd likely have to accelerate our timetable in complying with new rules."
Businesses have been keen for a transitional deal to avoid a sudden "cliff edge" change in relationships after Brexit, and the Prime Minister has acknowledged there may be a need for time to make "practical" adjustments.
Banks and financial firms wanting to trade with a country in the European Economic Area (EEA) must hold a designated passport, giving them licence to sell products to any other country within the EEA. Without access to a passport, UK-based banks would face a significant barrier when attempting to trade with countries within the European single market.
HSBC and Goldman Sachs said prior to the vote that thousands of jobs in the City of London could be moved to the Continent in the event of Brexit.
Mr Dimon added: "The British people have a complete right to decide what they want to do, as do all the people in Europe, and JP Morgan will be able to adapt to whatever the UK and the EU decide. But there was a good reason for the European Union, for peace and for the economic prosperity from the common market. Those are still two good reasons.
"I completely understand the frustration that it got bogged down with bureaucracy, and folks here in the UK would talk about it being anti-democratic, with people making decisions in Brussels who weren't elected that affected Britain. And the common market isn't everything, I completely understand that. Hopefully, rational heads will make it in a way that's good for everybody."