A boom in new financial technology jobs is helping London to offset a decline in traditional trading roles as the industry becomes ever more automated, though it may not compensate for Brexit-related losses in other financial sectors.
London's share of global daily forex turnover has rocketed to a record 43% - from 37% in 2016 - as it stole market share from New York and Asian hubs, according to a Bank of International Settlements survey last month.
London has long led in forex thanks to its convenient time zone and cutting-edge trading infrastructure.
But the news surprised many who had predicted Brexit would drive an exodus of banks and traders from London, or at least arrest its growth, while cities such as Hong Kong and Singapore were seen benefiting from a boom in local currency activity.
"London is hard to beat ... The depth and diversity of the skill pool here is unrivalled," Matthew Hodgson, the founder of Mosaic, one of a clutch of new fintech companies tapping into the forex industry, told Reuters.
Mosaic employed a handful of staff from a shared office around the time Britain voted to leave the EU.
"There is a network effect for talent, the FX liquidity and the clustering of institutions," he said, predicting London's advantage was "likely to have staying power" despite Brexit.
Big banks including Citi, BNP Paribas, Deutsche Bank, Goldman Sachs and UBS base their global forex heads in London.
And some banks have expanded their presence in recent years.
Dutch bank ING, for instance, chose London to centralise its forex trading operations, previously scattered across various cities. ING's global forex and rates boss Gary Prince said it was efficient to run more business out of the British capital.
London's forex fortunes could also be benefiting as European banks retrench from the United States, a recent HSBC report said.
The BIS gives no explanation for London's increased share.
Some traders caution that improved bank reporting of turnover - especially booming forex swap volumes - may have inflated the 2019 numbers globally, although that would not undermine London's success at the expense of rivals.
Part of the story appears to be new regulations that require investors to justify the prices they get while trading.
Rules introduced in 2018 by the EU aim to bring forex trading more in line with equities and have seen a raft of new London-based firms launch to provide "transaction cost analysis" to traders.
Their data, now used by 60% of buyside investors for currencies, according to Greenwich Associates, has reinforced what some market players already knew: trading costs are lowest when liquidity - the ease with which you can buy and sell - is deepest.
And that's overwhelmingly when Asian and American time zones overlap with London.
Quantifying intra-day liquidity is difficult without a single forex trading platform, but several traders said it had become easier and cheaper to transact in London than elsewhere.
Large investment funds can as much as halve execution costs when trading dollar/yen in London versus in Asia or New York, two senior traders told Reuters.
An extra bonus for the city is the increase in electronic trading and the use of computer models known as algorithms, or algos. Aimed at cutting costs and getting the best price, algos are often programmed to seek out the biggest liquidity pools.
"Because London has the liquidity and because liquidity is what larger players are looking for across the spectrum, they are moving the time in which they transact to the most liquid time zones," said Itay Tuchman, London-based global head of forex at Citi.
London's grip on the bank trading business is encouraging more forex fintechs to expand in the city. Often run by ex-bank traders and brokers, these startups offer a range of services, from data analysis to designing programmes that can shave milliseconds off trading times.
A City of London report said Britain's fintech sector employs an estimated 76,500 people, while London saw 61% growth in such jobs last year.
Some of Britain's most valuable fintechs are in the forex industry, such as TransferWise and Revolut.
"The network effect of having London as a talent hub to me is a very strong argument on why this city will dominate, even post-Brexit," Citi's Tuchman said.
Currencycloud, which builds forex payments infrastructure for businesses, has around three-fifths of its 250 staff in London and is expanding, its CEO Mike Laven told Reuters.
But is has also opened an office in Amsterdam and will employ 20 people there by the end of the year to help serve clients in the EU post-Brexit. These are jobs that "we would have hired in London," Laven said.
London's forex dominance may not offset a broader employment hit in financial services from Brexit.
That's because the same trends that are encouraging more centralising of trading are leading to forex automation and job cuts in the trading community.
At the world's 12 biggest investment banks, the number of trading and sales staff employed in bonds, forex and commodity globally was down to 1,400 by 2018 from 2,300 in 2010, analytics firm Coalition estimates.
But the draw of London remains strong, even for those who lose their jobs.
When Peter Kinsella was told in 2017 by Commerzbank that it was shrinking teams and might move his forex strategy role from London to Frankfurt, he left the bank.
"You've got the best people in London and the best market information. You can get a call on a Monday and someone will ask what are you doing tomorrow morning We've got a meeting with a politician on Brexit," said Irishman Kinsella, who now advises well-heeled clients at Swiss bank Union Bancaire Privee - from London.
"That type of connectivity you don't get in other parts of the world."
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