LONDON Some of London's brightest financial technology talents have begun weighing a future elsewhere, worried Britain will lose its hard-won position as an industry hub if it opts out of the European Union in four months.
London has worked hard to cultivate its reputation as a top global location for so-called "fintech" firms -- technology startups that aim to compete with traditional banking and financial services, such as money transfer company TransferWise, crowdfunding platform Crowdcube and peer-to-peer lender RateSetter.
The British capital is in a fierce race with San Francisco, New York, Berlin and Hong Kong to remain a leader in the fast-growing sector, an ambition outlined by Finance Minister George Osborne in 2014.
But as a June 23 referendum on British membership in the EU nears, the industry is worried London will lose momentum and its reputation for innovation if Britain has to renegotiate its trading relationship with the EU or deal with economic fallout from an "Out" vote.
Seven out of 10 London-based fintech companies interviewed by Reuters said they might move their headquarters. Two said they would definitely stay, and one declined to comment either way. All 10 said British exit from the EU, or "Brexit", was a serious concern for their businesses.
"I don't think there's a contingency plan, which is part of my personal concern," Eileen Burbidge, a partner at venture capital firm Passion Capital, which has backed corporate information company DueDil, bitcoin exchange Coinfloor and online payments firm GoCardless.
"I think there would be a bit of a reset of people, offices, HQs and activities if Britain were to leave Europe," added Burbidge, who also serves as the British Treasury's envoy for fintech and chairwoman of government agency Tech City UK.
London Mayor Boris Johnson has been a particularly enthusiastic booster of fintech, which marries the city's longstanding strength as one of the world's top financial capitals with its more recent success as a tech startup hub.
He has since emerged as one of the leading voices calling for Britain to leave the EU. A spokesman for Johnson declined to comment for this story. In general however, campaigners to take Britain out of the European Union say threats by companies to leave London are overblown.
"It's absolutely ridiculous. Companies said they'd do this if we didn't join the euro, and how many of them up and left when we didn't?" Jordan Ryan, a spokesman for "Out" campaign group Leave. EU, said. "Nobody's going anywhere. It happens every single vote."
Britain's London-focused fintech sector was the biggest in the world last year, earning 6.6 billion pounds in revenue to beat California and New York, according to a report by accounting firm EY commissioned by the British government.
While that is still a pittance compared to the revenue of traditional banks -- on its own JP Morgan Chase brings in 10 times as much -- the nascent sector is one of the fastest growing in the world.
More people work in fintech in Britain than in Singapore, Hong Kong, Germany and Australia combined.
Investors injected over $700 million into British fintech last year, the report said, encouraged by accommodating regulation and the ease of doing business in other European markets from London.
But startups that had flocked from all corners of the world are nervous about losing access to European customers, tech talent and funding.
Members-only fintech association Innovate Finance said a survey showed 82 percent of firms wanted Britain to stay in the EU, believing Brexit would be a "disaster" and isolate London from the wider industry. About a quarter of the London-based trade group's member firms are run by CEOs from other EU countries.
"Would there be a flight of capital out of the UK? Would investors be spooked? All of it is so uncertain that it's dangerous," said Dan Gandesha, CEO of real estate crowdfunding platform Property Partner.
Some consider English-speaking Dublin or multi-lingual Luxembourg potential successors to London's fintech mantle in Europe, but a German city seems most likely as fintech firms there raised about $550 million last year, only slightly less than in Britain.
"Other cities such as Berlin or Paris would start to look more appealing than London as a European hub," said Taavet Hinrikus, chief executive of TransferWise, which is backed by British billionaire Richard Branson.
"I hope we don't have to make that decision, but yes we are considering our options at the moment."
Firms whose business models rely on being part of the EU are particularly worried. Monese, which provides European immigrants to Britain with bank accounts "in minutes" via a mobile app, has begun thinking of post-Brexit options as the number of migrants could sharply decrease.
"We would rather it didn't happen," Monese Vice President of Growth Mulenga Agley told Reuters, saying the company might decide to shift its main business from Britain to mainland Europe and focus on customers migrating to and within the continent.
"(Brexit) would make us seriously consider whether we should be in the U.S., rather than the UK," Property Partner's Gandesha said, pointing to reduced growth opportunities in Europe.
Other firms expect Brexit to upset operations, but not enough to make them move.
International payments company Currency Cloud said it would stay in Britain as it needed to keep strong ties with what it sees as the world's unmatched foreign exchange capital. The firm is backed by Sapphire Ventures and Japan's Rakuten.
CEO Mike Laven said, however, that as Brexit would raise costs and inhibit growth, he would probably have to focus more on growing the firm in Europe to "keep things going correctly".
"Would (London) be the right place for all of our jobs? I'm not sure."
(Reporting by Richa Naidu, editing by Sinead Cruise and Peter Graff)