Media playback is unsupported on your device Media caption Jamie Dimon says 4,000 UK jobs could go

The chief executive of JP Morgan has told the BBC it could cut its 16,000 UK workforce by more than a quarter if financial rules diverge after Brexit.

Jamie Dimon said the US bank had not needed to make drastic cuts on day one after the EU referendum.

But he revised his long-term estimate of job losses upwards if Brexit talks failed to produce an outcome close to the current arrangements.

Mr Dimon added that such a scenario would harm London as a financial hub.

The boss of the US's most valuable bank had warned in the run-up to the referendum that 4,000 jobs could go if the UK voted to leave the EU.

Since then, JP Morgan has revised that estimate down to between 500 and 1,000 jobs, leading many to dismiss his warnings as part of "Project Fear".

In an interview with the BBC at the World Economic Forum in Davos, Mr Dimon acknowledged that on closer analysis, it turned out the bank did not need to make such drastic moves on day one of Brexit.

But he added that could go up again. "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 4000," he said.

Macron charm

When asked if that outcome would represent a real threat to the future success of London as a financial centre, he gave a single word answer: "Yep."

A government spokesperson said it was determined to maintain London's "competitiveness now and in the future".

That includes a partnership with the EU "based on our rules and regulations being the same at the start and on our shared belief in free trade and a commitment to high regulatory standards".

But if Jamie Dimon's warning comes to pass, where would these jobs go?

JP Morgan, along with fellow US bank Goldman Sachs, has already indicated that Frankfurt will be the major beneficiary.

But in common with many chief executives at the Swiss ski resort, Jamie Dimon is increasingly impressed with the offer from the new darling of Davos, French President Emmanuel Macron.

Macron invited Mr Dimon, along with over a hundred international chief executives, to the Palace of Versailles this week and the charm offensive appears to be working.

Image copyright Getty Images Image caption Emmanuel Macron told Davos: France is back

According to Mr Dimon, it's not just style but substance.

"Here's a guy who is 39 years old who formed his own party and within a year won a parliamentary majority," Mr Dimon said.

"He spoke and answered questions for two hours and was pro-business, pro-capital, pro-reform.

"It's a shocking change from what came before and if Macron had been around before maybe you would have had the British staying in," he added.

That sounds fanciful to UK ears, but the positive impact the new resident at the Elysee Palace has made on business leaders in Davos is hard to over emphasise.

'Kick-ass'

Of course, the main event for many is yet to come - the arrival of Donald Trump who will give the closing address on Friday.

Trump is the first sitting US president to attend Davos since Bill Clinton and his appearance is all the more surprising given his previous public scorn for the gathering of the pro-globalisation elite it attracts.

His speech has been described as "kick ass" by his advisers.

Global leaders including Narendra Modi of India and Justin Trudeau of Canada have used their time on the platform to warn against Trump's protectionist America First approach.

Image copyright EPA Image caption Donald Trump is set to speak at Davos on Friday

In Jamie Dimon, the President has a supporter amidst a fairly hostile crowd.

He described Trump's complaints about global trade and the recent imposition of new tariffs on solar panels and washing machines as "shots across the bows" signalling legitimate concerns.

"When America talks about free trade, it means reciprocal trade and we don't have that. Foreign companies can buy 100% of US ones but not the other way, tariffs are unequal and I think he's right to raise that."

It's worth remembering that Trump's recent corporation tax cuts from 35% to 21% will create a windfall of $20bn for JP Morgan over the next five years.

Dimon has pledged that money will be used in part to increase wages for lower paid workers in the retail part of JP Morgan Chase and to increase lending to lower income borrowers.

He told the BBC that the bonus pool for the high rollers at the investment bank would not increase as a result of Trumps tax largesse. That will all be worth monitoring but if correct is the kind of "trickle down" that will get two thumbs up from the White House.

Jamie Dimon is not a supporter of Trump on all trade issues, and would hate to be called a protectionist, but if we are going to get a punch up on Friday with Trump in one corner and the rest of the world in the other - it's clear which corner Jamie Dimon will be in.