New York and London function as two prongs of one global economy. Banks and other financial companies headquartered in New York usually have their second biggest offices in the British capital, and vice versa.

For years, that’s made economic sense. For London-based companies, New York provides an unparalleled density of financial firms, a regulatory framework in which to do business, and access to non-European markets. London provides much of the same for New York-based companies who need access to European markets.

Unfortunately – for London, at least – Brexit could change all of that: an isolated UK could mean financial firms would have a hard time accessing and doing business with other European markets. And while several EU rivals, from Frankfurt to Paris to Madrid to Amsterdam, are waging campaigns for London’s financial businesses, New York – with its already established financial sector and finance-friendly regulatory environment – could get the majority of Brexit’s financial runoff, according to some experts.

And this has New Yorkers bracing for a wave of British capital that could affect not only the financial industry but the entire city, from cultural production to housing.

“People – financial people, consultants, bankers – already started calling looking for apartments two or three months ago,” says Gennady Perepada, a real estate consultant who specialises in helping foreign millionaires and billionaires buy apartments in New York. “Any problem that’s not in New York is good for New York.”

London and New York’s financial industry rivalry goes back decades, and the two cities jockey for the title of biggest financial centre each year. According to Z/Yen, a London-based business think tank, London currently outranks New York by just one point on their scale. The next financial centre, Singapore, is 42 points behind New York.

The New York Stock Exchange on Wall Street. Photograph: Spencer Platt/Getty Images

In other words, New York and London stand alone as centres for global finance, far ahead of the competition. That’s for a few simple reasons: both have a tremendous density of talent, they house large groups of ancillary financial service professionals – lawyers, accountants, consultants – and, most importantly, their clearing houses (the places where investors and sellers can trade in complex financial instruments) are the world’s most developed, meaning London and New York are the only places where all of the world’s major currencies can be traded.

“The UK has over one million people employed in finance,” says Vincenzo Scarpetta, a senior policy analyst at the think tank Open Europe. “The whole city of Frankfurt, by comparison, has 725,000 inhabitants. So there are only a few global centres where the industry can really go.”

Indeed, New York is such an obvious choice for capital fleeing from London post-Brexit that it seems, unlike other European cities, it hasn’t had to move a finger to convince British investors to consider taking the leap.

“I’ve talked to CEOs who are being heavily wooed by Paris, Amsterdam, Dublin, Frankfurt. They’ve sent out delegations, had formal presentations,” says Kathryn Wylde, CEO of the Partnership for New York City, which represents and lobbies for the financial industry and other corporations here. “Is New York doing anything similar? No.”

Wylde and others say it’s because New York already has more immediate advantages: a larger talent pool than any of those cities plus more English speakers, and a pre-existing regulatory system for complex financial transactions such as derivatives.

But it’s also because any benefit for New York will take a while to materialise, so there’s no rush to woo financial firms. Wylde envisions a slow bleed from Britain, not a flood: the majority of jobs in financial services are mid-level jobs, she points out, and the expensiveness of New York makes it unlikely companies would uproot their support and administrative staff for its shores.

Rents have already skyrocketed in New York. Photograph: Pacific Press/LightRocket via Getty Images

Instead, she says, headquarters of financial institutions that need to attract top international talent would be the ones relocating – but slowly. “These are long-term implications that really depend on how Brexit shakes out.”

In New York, however, nearly every industry is tied into finance, and the industries most closely associated with it – such as real estate – are already feeling impacts post-Brexit, albeit at a low level.

Trump is worse for the global economy than Brexit, so they kind of balance each other out Richard Florida

“Money-seeking New York or London will now fall in New York instead,” says Will Silverman, managing director of investment sales at Hodges Ward Elliott, a commercial real estate investment firm with offices in London and, as of recently, New York. “But it’s probably actually less impactful than people thought it would be, especially if Brexit takes forever, and is walked back at all.”

Still, some New Yorkers are worried about what global market fluctuations will do to the city. New York’s theatre scene, for example, is heavily reliant on British and other foreign capital.

“Whenever there’s a global event, investors and consumers freeze up and stop reaching for their wallets,” says Ken Davenport, a long-time Broadway producer. But there hasn’t been a freeze yet, and he says that even with Brexit, people need to be entertained, so he’s not too worried.

“I’m more worried about what Brexit will do to the West End than to Broadway,” he adds.

The biggest concern, it seems, comes from New York’s most vulnerable, who have been increasingly destabilised by the city’s globalised economy. Rents in New York have skyrocketed in the last decade, and that means any new wave of capital fleeing Britain and entering New York could put further pressure on previously poor neighbourhoods already feeling a housing crunch, leading to even more evictions and rent increases.

In Brooklyn for example, many condo projects rely on billions of dollars from foreign investors seeking to place their money in economies more stable than those of their home countries. If the UK’s economy destabilises because of Brexit, there could be even more capital finding its way into buildings in vulnerable neighbourhoods.

“A lot of the rent-stabilised buildings here are being bought up by foreign investors,” says Imani Henry, an anti-gentrification activist in the Flatbush neighbourhood of Brooklyn. “They’re being wooed with citizenship and tax breaks, and meanwhile we have entire blocks of businesses closing because of high rents.”

Ironically, for those nervous about the effects of Brexit, Donald Trump may yet prove their saving grace. His election as the 45th US president may destabilise its economy enough to overwhelm any effect that capital from the UK could have on New York.

“If the UK has excluded itself from the world economy, New York will gain. That was my first thought, until Trump was elected,” says Richard Florida, director of cities at the Martin Prosperity Institute at the University of Toronto. “Trump is worse for the global economy than Brexit, so they kind of balance each other out.”

Follow Guardian Cities on Twitter and Facebook to join the discussion