London's financial district | Dan Kitwood/Getty Images Brexit financial shock today, and for years to come The UK’s financial industry is likely to struggle and employ fewer people.

LONDON — The world of finance will never be the same after the historic decision by the U.K. to leave the European Union.

With markets crashing around the world, an economic shock looming in Britain and possibly the globe, and the prospect of losing London as their European headquarters, financial executives ended their all-night vigil on Friday morning with the realization that their industry was likely to struggle for years, employ fewer people, and find it more difficult to operate across national borders.

"I have never been so depressed," said a senior financial executive after enduring a topsy-turvy referendum night that ended in the shock Brexit decision. "This is a massive own goal. Our industry will shrink, the markets will suffer, and the future looks very very uncertain."

With the U.K. likely to lose access to the EU's single market — and to the lucrative market for clearing euro-denominated securities — many banks will have to move jobs and operations to what the Brits call "the Continent" at great expense.

The net result will be a smaller, more divided sector, according to bankers.

Investors are already wondering whether the BoE will have to raise interest rates to stem the slump in the pound.

One exhausted executive on Friday morning estimated that his firm would end up cutting 15 to 20 percent of all its jobs in Europe once it's all said and done. Another said that setting up shop in a different EU country, as most financial groups will have to do, could cost an additional $30 billion (€26 billion) in capital.

Reassurances against panic

But the first order of business for both executives and policymakers will be to deal with the market rout caused by the referendum vote.

Two types of intervention are likely in the coming hours and days: verbal reassurances by leaders and central bank governors that the authorities stand ready to help both markets and the world economy; and more concrete moves to stabilize markets that are deeply unsettled and are showing signs of panic.

On the first front, Japan's central bank Governor Haruhiko Kuroda already tried to soothe the markets' nerves on Friday morning as did Martin Schulz, the president of the European Parliament. The Bank of England also stepped into the fray, with a statement saying that it would "take all necessary steps to meet its responsibilities for monetary and financial stability."

"Large institutions in the U.K. that rely on passports are going to be looking very carefully at market access" — Phillip Souta, head of U.K. public policy at the law firm Clifford Chance

But with the pound plunging, U.K. and European equities markets under severe stress, and even the U.S. Dow Jones Industrial Average projected to open at a huge loss, more will be needed -- and soon.

On the more concrete front, the BoE and the European Central Bank will have to make good on their promise to step in to provide markets with additional liquidity to avoid any snafus in trading that could exacerbate losses and tip hedge funds or even banks over the edge.

But that may not be enough.

Investors are already wondering whether the BoE will have to raise interest rates to stem the slump in the pound or whether a concerted monetary effort by the Big Four central banks — the U.S. Federal Reserve, ECB, Bank of Japan, and BoE — would be needed. That would bring back memories of the direst days of the financial crisis of 2008, when the monetary authorities had to stand together in a desperate, but ultimately successful, effort to avoid a global depression.

Beyond the immediate market reaction, the future looks bleak for global finance and its titans. The first casualty of Brexit is almost certain to be the City of London, which employs around 400,000 people in its two incarnations of the Square Mile and the gleaming skyscrapers of Canary Wharf. Collectively, these workers and their firms contribute more than 8 percent of the U.K.'s economy and have been behind the rise in markets such as property, hospitality, and entertainment.

London' s single biggest attraction to global financial firms — the "passporting" rights that enabled them to operate across the 28 EU member countries from a single base in London — is likely to disappear at the end of the break-up negotiations between the U.K. and Europe.

That's a big blow to U.S. banks like Goldman Sachs, J.P.Morgan Chase, and Citigroup, which have taken advantage of London as their European launching pad to reach a single market of some 500 million people with an economic worth of around $19 trillion.

"Large institutions in the U.K. that rely on passports are going to be looking very carefully at market access," said Phillip Souta, head of U.K. public policy at the law firm Clifford Chance.

People close to the situation said all banks had made contingency plans that involved using an existing legal entity on the Continent, or setting up a new one, to continue to benefit from the EU single market.

But they also cautioned that it would take time and money to uproot thousands of jobs from London, inject capital into the new entity, and rewrite reams of contracts and legal documents.

"We pray that there is no rush in this," said one financial executive. "Our ideal situation would be a five-year transition period."

That would be considerably more than the two-year negotiation period provided for under Article 50 of the EU treaties. "Our base-case is that this will take longer than two years," said Clifford Chance's Souta.

The City's cheerleaders will try to keep their chins up and demand the best possible deal.

Even with contingency plans, legal advisers at the ready, and the belief that the world will always need bankers, it was difficult for many financial executives to hide their shock.

"Financial services contribute £66.5 billion in taxes to the Treasury -- 11 percent of total government receipts -- and City businesses I have consulted believe they must be allowed access to the single market without discrimination," said Mark Boleat, the chairman of the City of London Corporation, the local authority for the Square Mile.

EU won't budge

That may prove wishful thinking, though. Much will depend on when those negotiations start and what tone the EU decides to adopt towards the exiting Britain.

On that front, financial executives weren't very hopeful. "The initial tone would be conciliatory, but Europe is obliged to take a tough negotiating stance with Britain to discourage other countries from doing the same," said one financial insider. "This will force every bank to ask itself: Am I a domestic bank or an international bank?"

On one issue, the EU is not expected to budge: London will lose the right to clear euro-denominated securities and foreign currency trades. That loss, which will probably benefit Frankfurt, will reduce the British capital's role as the trading post of global finance, the place where Asia, the U.S., and Europe went to do business when they needed to interact with each other.

Many executives said London would not be completely hollowed out, at least not until the EU talks are completed. But it would go back to being a much smaller version of the current financial megalopolis. "It will be like going back to the 1980s and 1990s," said a banker. "New York is the big winner here."

Even with contingency plans, legal advisers at the ready, and the belief that the world will always need bankers, it was difficult for many financial executives to hide their shock on Friday morning at the British voters' decision.

"I never thought they would," said a battle-hardened veteran of many crises and market routs. "I never thought they would."