A shortage of prime office space in rival European financial centres may torpedo plans by some of London's banks to quit the UK capital if Britain opts out of European Union membership.
Prime Minister David Cameron has promised to renegotiate Britain's relationship with the EU and then hold a vote by the end of 2017 on whether to stay in the bloc or leave.
Deutsche Bank (DBKGn.DE), the euro zone's second largest bank by assets, confirmed this week it was considering cutting its UK operations if the country pulled out, and other big global banks are expected to rethink the scale of their UK operations under such an eventuality.
But decamping from their expansive London homes to Frankfurt, Paris or Dublin would be no simple task.
"None of the major European cities could cater for that kind of demand at the click of fingers or even with six months notice," said Mat Oakley, head of commercial research at global real estate consultancy Savills (SVS.L).
"The markets across Europe are pretty tight in terms of vacancy rates and lenders are so risk averse in terms of speculative office development that a bank would have to do a pre-let (commit to rent a property before it has been built), or buy a site," he said.
Both of those options would incur large unnecessary costs if the UK stays in the EU.
"The potential referendum on EU membership bothers many of our clients immensely," said Miles Gibson, Head of UK Research at real estate advisor CBRE (CBG.N).
"Even though we think the PM would campaign for an 'in' vote and would ultimately win, the sooner he can resolve this issue, the better."
London's status as a global financial centre dates back to the 16th century, when it became a hotbed of merchant banking, international trade and commerce strategically placed between the 'Old World' Europe and the 'New World' Americas.
Fortune-seekers have flocked to the capital in every century since and a large financial services industry now thrives in two main financial districts - the historic City and the modern estate of Canary Wharf, built in the late 1980s to accommodate banks keen to cash in on an era of light-touch regulation.
London was home to 143,500 banking professionals at the end of 2014, according to lobby group TheCityUK.
Office occupiers in the UK capital typically allow 11.3 square metres per workstation, according to the British Council for Offices.
London based banks tend to occupy this size of space in large buildings where trading and lending teams can work in close contact. So if they wanted to move 100,000 of staff out of London, they would need to find 1.13 million square metres of equivalent prime space elsewhere.
But there is just 1.46 million square metres of vacant prime and non-prime quality office space in Frankfurt, 941,000 square metres in Paris and 451,111 square metres in Dublin, data from real estate firm CBRE shows.
Some banks have already shifted back-office and support functions to countries like Ireland.
But Dublin is not an easy option for any substantial future move either, having seen the largest annual increase in prime office space occupancy costs - a proxy for demand - worldwide, recording a 34.9 percent rise year-on-year in 2014, CBRE said.
Just 1.8 percent of the city's total Grade A office space - prime real estate in its two best postal districts of Dublin 2 and Dublin 4 - was available to rent at the end of the first quarter 2015, further data from the real estate advisor showed.
As a result, says Savills' Oakley, many of his financial services clients have suggested they are likely to leave the bulk of their operations in place whatever the referendum outcome.
"...The practicalities of hiring or moving 1,000-5,000 staff to comparatively small markets in Ireland, France or Germany were so spectacularly difficult, they couldn't envisage moving lock, stock and barrel," he said.
"NOT ALL OR NOTHING"
While other banks may be concerned about the impact of Britain leaving Europe, many of them are pressing ahead with building projects in London that would appear to secure the city's position as Europe's top financial centre.
Goldman Sachs, which employs around 5,500 people in Britain, is building a new 111,500 square metre European headquarters in London's Farringdon business hub.
Separately, construction is underway to create a new 65,000 square metre City base for Swiss bank UBS (UBSG.VX).
And even Deutsche Bank, despite its worst-case scenario contingency plans - is looking to rent additional space in Canary Wharf to house people from several London offices, according to a source with knowledge of the matter. Deutsche employs around 9,000 people in the UK in 16 locations, the bulk of which reside in its Moorgate headquarters.
"It's not as binary as just simply moving out of the UK altogether," said one investment banking source speaking on condition of anonymity.
"There are many reasons why banks are in the UK. Their clients are here, it has the biggest stock exchange in Europe. It's not all or nothing."