Jamie Dimon, the chief executive of JP Morgan - which employs 16,000 people in Britain - has already said he will move hundreds of people out of the UK when it leaves the EU to Dublin and Frankfurt where it already has operations.
Today in Paris he went further.
He said there had been too much focus on what would happen on day one after Brexit. Ominously for the UK, he said that how many more jobs would go to centres all around the EU was no longer in the UK's control.
"What happens next is totally up to the EU, it's not up to Britain. And so once you have that first step, if the EU determines over time that they want to start to move a lot more jobs out of London into the EU, they can simply dictate that. The regulators can dictate it, the politicians can dictate it."
France's former President Francois Hollande once labelled finance "the enemy". Today the Prime Minister Edouard Philippe - speaking at the same event - promised that era was over.
Lower personal and corporate taxes, making it easier to hire and fire, English law courts conducted in English, and three brand new English-speaking schools are among the incentives on offer to bankers prepared to relocate.
Another heavyweight banker, HSBC chief executive Stuart Gulliver, welcomed the proposed reforms.
He has already said he will move up to 1,000 bankers to Paris but he also expressed some reservations - saying it would take time to forget how reviled they were in France.
But under new President Emmanuel Macron it seems France means business with its pro-banker message and as Jamie Dimon said today, his ears are wide open.

The 30-year-old from Holywood, 32nd in the marathon at the Rio Olympics, took ninth place in the Welsh capital.
Pollock finished with a time of 65.53 in a race won by Shadrack Korir of Kenya in a course record of 60.54.
Medical doctor Pollock was returning to Cardiff where he ran a superb 62:46 to finish 14th at the World Half Marathon Championship.
In July, Pollock was the top Irishman in the half marathon at the European Athletics Championships in Amsterdam as he took 17th place with a time of 1:04.28.
Last weekend, Pollock ran his first race since the Olympics when he won the Aurora 10k in Bangor.

The fishermen aboard the St Apollo escaped into their life raft during the incident early on Monday.
A rescue helicopter was sent from Prestwick but the group were reached first by the Oban lifeboat at about 03:30.
They were taken ashore at Oban. It's thought no one was been injured.

Speaking to BBC's Nolan Show, the 53-year-old entertainer from Donegal said nerves had impacted his performances.
He said his anxiety was sometimes so bad he had struggled to walk down the stairs before the live show.
Mr O'Donnell became the third celebrity to leave the competition at the weekend after failing to wow the judges in the dance off.
He and Kristina Rihanoff faced sports presenter Kirsty Gallacher and Brendan Cole.
But mistakes saw him being sent home.
Mr O'Donnell and his partner performed the American Smooth to Frank Sinatra's Fly Me to the Moon, while Gallacher and Cole performed a Paso Doble to U2s Beautiful Day.
Speaking on Saturday night's show, judge Craig Revel Horwood said: "One couple did really, really well in the dance-off and one couple, I am afraid, made a lot of mistakes.
"I am going for the couple that was the most powerful, and that was Kirsty and Brendan."
Mr O'Donnell told the Nolan Show that his nerves took him by surprise.
"If I didn't have to do that live show every Saturday night it would have been the perfect experience.
"The strange thing is I hadn't anticipated being nervous, I thought it would be hard to learn the routines, but I never anticipated that kind of nerves.
"I can't explain it. It's ridiculous from the point of view of someone who has been on stage as many times as me."
He told the show he couldn't speak highly enough of the crew, dancers and judges on Strictly but said he was "relieved" to have been voted off.
"The dance off was a disaster". He said.
"I forgot everything and maybe I was quite happy to go to tell you the truth."
Mr O'Donnell said he wouldn't be doing any more reality TV, but he would keep dancing.
In capital cities around Europe there is a smell of blood in the water - a sense that the UK's financial services industry has been wounded by Brexit and that presents a chance to take a bite out of its dominant position.
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Northern Ireland's top distance runner Paul Pollock claimed a top 10 finish in Sunday's Cardiff Half Marathon.
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Five people have been rescued after their fishing vessel ran aground and capsized in the Sound of Mull.
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Irish singer Daniel O'Donnell was "relieved" to be voted out of Strictly Come Dancing at the weekend.