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A vote to quit the European Union would trigger a flood of up to 100,000 banking jobs out of London, senior City figures have warned.

One board-level banker told the Standard that “Brexit” would force dozens of banks that have their European headquarters in London to relocate highly-paid professionals to rival EU financial centres such as Frankfurt, Luxembourg and Dublin.

He estimated that staffing levels at some investment banks in London could fall by as much as half, with a huge knock-on effect for the broader economy and property prices.

The banking chief, who did not want to be identified, said: “I don’t think people here have any idea just how much the other European centres would love to have the jobs and tax revenues we bring to London.

"We get approaches all the time from Europeans saying ‘please move here, we’ll give you tax breaks’.”

His warning is one of a growing number from the City’s “top brass” about the potential threat to jobs if Britain votes to pull out of the EU.

David Cameron has promised that the referendum will take place before the end of 2017 but many commentators now believe it will be called by next autumn.

More than 250 foreign banks are based in London and they contribute about half the tax paid by the financial services industry in Britain.

Mark Boleat, policy chairman at the City of London Corporation, said: “If as a country we were to vote to leave, then London’s position as a leading financial centre would remain but without doubt there would be an impact on our relative size and the jobs we support.”

Confidential client research from analysts at US investment bank Morgan Stanley, seen by the Standard, warned that “firms for whom the EU market is important” would need to “adjust their footprint” in London if the Eurosceptic cause was victorious.

Sir Mike Rake, deputy chair of Barclays and chairman of BT, said: “It is extremely difficult to quantify the number of jobs that would be lost and the time frame over which that might happen but leaving the EU would severely damage London’s competitiveness and our financial services sector.”

There have been growing hints from financial institutions that they are starting to plan for Britain quitting the 28 member club.

Both HSBC, which announced a review of the location of its global headquarters in April, and JP Morgan are reportedly in talks about moving sections of their businesses to Luxembourg in part because of the threat of Brexit.

Deutsche Bank, which employs 9,000 people in Britain, has set up a working group to review whether to move parts of its business from Britain in the event of a UK withdrawal.

US asset management group Vanguard, which has a City office, has admitted that Brexit would have a “significant impact” on its operation across Europe and has already started planning for it.

Many senior bankers are concerned that they would lose the financial services “passporting” rights enjoyed by fellow EU members.

Most mainstream City analysts still believe that a vote to leave is unlikely. But if it did happen it could be followed by the biggest drain of jobs from London since the Big Bang reforms of 1986 and undermine the City’s status as the financial gateway to the world’s largest economic bloc.

Earlier this month Thierry Lesage, partner at Luxembourg law firm Arendt & Medernach, told the Bloomberg financial news services: “We are seeing real interest from London-based institutional clients for solutions offered in Luxembourg in case of Brexit.”

However, not all fear that Brexit will be bad for the City. Axel Weber, chairman of Swiss bank UBS, has said that leaving the EU “won’t undermine the UK as a financial centre” and could be negotiated so that the City can still have “favourable” access to European markets.

A recent poll conducted by the investment banking “trade body” the Chartered Institute for Securities & Investment found that around three quarters of its member feared the impact of Brexit with one respondent saying “It would be a disaster for all financial specialists in London.”

The institute has now organised a series of public debates on the issue it has called the “The Great British Break-Off” to raise awareness of its implications.

The first one took place earlier this week at the offices of accountants Grant Thornton with speakers including Boris Johnson’s economic adviser Gerard Lyons. A second is scheduled for 26 January.