JP Morgan, one of the world's biggest banks, employs 16,000 people in the UK. Not all of them are highly paid investment bankers in London.
At an event later today in Bournemouth (where the bank employs 4,000 people), Jamie Dimon, its chief executive, also known as the "King of Wall Street," appearing alongside Chancellor George Osborne will warn he may have "no choice" but to reduce the bank's UK headcount and shift jobs to Europe if the UK votes to leave the European Union.
A renowned deal-maker himself, he's expected to say that a vote to leave would be a terrible deal for the UK economy. His comments on the role of London will be listened to closely.
As a member of the EU, the UK serves as a base for foreign owned banks to sell services to any other country in the EU. On Brexit, that so-called "passport" for the selling of financial services may have to be renegotiated.
Other EU financial centres would certainly welcome the chance to prise lucrative business away from London. Some, including one of his own former lieutenants (pro-remain Barclays chief executive Jes Staley) say London's crown as Europe's most important financial centre would not slip even if there was a short-term impact on the UK economy.
Enlisting the support of one of America's most famous investment bankers could be seen as a risky strategy. He may be well-known on Wall Street and a major employer in the UK, but some will question whether his intervention will do much to change hearts and minds across the country.