Most senior executives in the City of London appear to side with the Remain camp in the lead-up to Britain's in-out EU referendum on June 23.
But the outcome of the referendum is far from certain, and many financial institutions are examining the implications of a win for the Leave camp, despite most polls giving EU loyalists a slim lead.
With analysts saying a Britsh EU exit, or Brexit, could cost the country some 100,000 financial services jobs amid stalling economic growth and a falling pound, some firms have started making contingency plans for Britain's life outside the EU.
Deutsche Bank has set up a group to examine whether to move parts of its London business and some of its 9,000 British-based staff after a Brexit.
Jamie Dimon, JP Morgan's chief executive, told the Financial Times that his bank would reduce its London operations after a Brexit: “If we can’t passport out of London, we’ll have to set up different operations in Europe,” Dimon said.
In a survey of 101 senior executives last year by TheCityUK, which represents financial firms in London, 37 per cent said their companies were "fairly likely" to move some staff from Britain to other locations in Europe after a Brexit.
And a survey last year by consultants EY found that 31 per cent of investors planned to freeze or reduce investments in London until after the vote.
"I'm getting calls left, right and centre from banks who are worried," says Simon Hix, a political scientist at the London School of Economics.
Hix says the banks are mainly concerned about two issues: whether a post-Brexit Britain could retain access to the EU's financial services "passport," and whether the city could continue its services in euro-denominated products.
The passport allows a firm authorised in a European Economic Area (EEA) state to market its services in any other EEA state through local branches or cross-border business as part of a single market.
Hix says Britain's retention of the passport "would be possible if the UK stayed in the EEA but would be very unlikely if the UK left the single market and had to negotiate a free-trade agreement with the EU."
"My understanding is that no FTA [free trade agreement] the EU has signed so far has much coverage for financial services, and I don't see much appetite in Frankfurt or Paris for allowing the City of London to access the single market if the UK leaves the EU," he told dpa.
Hix also believes it is unlikely that a post-Brexit City of London would be allowed to continue trading euro-denominated products, with that business moving to other EU nations.
Chris Martin, an economist at the University of Bath, says the financial services passport is "a major reason why many foreign banks, especially American and Swiss ones, have large UK-based subsidiary operations."
Investment banks Goldman Sachs and JP Morgan told a parliamentary commission that the passport system was key to their choice of London as a base for their European businesses.
"The presence of these large banks then encourages Eurozone-based banks like BNP-Paribas and Deutsche Bank to also base a large part of their operations in the UK," Martin wrote in a recent analysis.
Martin warns that London's dominant position in wholesale banking is "already under threat" as Britain struggles to protect its status as a major market for euro-denominated assets while remaining outside the Eurozone. But a Brexit would make the outlook for the City's euro-denominated business "much bleaker," he said.
While a Brexit could also damage Britain's regional centres for financial services, London would be hit hardest - its banking sector especially, according to a government report backed by Prime Minister David Cameron and Chancellor George Osborne this week.
The Treasury analysis says more than 5,000 British firms involved in banking, investment and insurance have financial services passports.
A report last month by consultants PwC for TheCityUK estimated that Britain could lose up to 100,000 financial services jobs and up to 12 billion pounds in annual revenue by 2020 after a Brexit, although it said the industry would partially recover by 2030.
Nearly 60 per cent of dozens of bankers polled by the British Bankers' Association in March said Brexit would have a negative impact on their business, while 26 per cent said the impact would be "significant."
"The single market is of crucial importance to the UK banking industry, which employs over half a million people, contributes over 31 billion pounds in tax a year, and is the country's biggest export industry," said Anthony Browne, the banking association's chief executive.
But the debate in London is not all one way. The pro-Brexit Business for Britain group mobilized more than 100 senior executives, many from hedge funds and small banks, who argued in a letter to London's Evening Standard newspaper last month that Britain's financial services would flourish in a less-regulated environment outside the EU.