"We are going to use the three banks we already have in Europe as the anchors for our operations," Daniel Pinto told Bloomberg News.
"We will have to move hundreds of people in the short term to be ready for day one, when negotiations finish, and then we will look at the longer-term numbers."
UK-based banks and other financial firms face losing "passporting" rights to sell services to clients operating in the European Union once Britain definitively quits the EU in March 2019.
Talks between the two sides are not set to begin until after a UK general election slated for June 8, at which Prime Minister Theresa May is expected to be returned with an increased majority.
Many banks are now looking to cities in the eurozone to set up a European presence supervised by the European Central Bank.
JPMorgan Chief Executive Officer Jamie Dimon warned before last year's referendum that as many as 4,000 British jobs could be relocated.
Deutsche Bank (IOB: 0H7D.IL - news) said last week it could move up to 4,000 jobs away from Britain, while Goldman Sachs (NYSE: GS-PB - news) has said it expected to begin relocating positions next year.
"We have looked at a number of locations but the choice of Frankfurt was very natural," bank chairman Jose Vinals said.
Estimates for the possible number of jobs to be lost by London's key financial sector range from as few as 4,000 to as many as 232,000.