Up to 70,000 finance jobs could be lost in London if the UK loses access to the European single market, according to a new report by thinktank Centre for London.
The report 'Open City: London after Brexit' looks at the challenges Brexit poses for London's position as a global city.
Focusing on the financial sector, the report said a lack of access to the single market would be a "significant blow" for London.
"Our global city service sectors need urgent clarity on how access to the single market will be regulated. The risks to London's economy, from the loss of these workers and students, and access to the European single market, are considerable. 70,000 jobs could be lost in financial and related services alone."
The report said financial and business services account for around 50% of London's exports and, as of 2013, fund management and securities contributed £8.5bn in exports. Meanwhile, monetary finance contributed £24bn, according to statistics from the Greater London Authority used in the report.
Prior research by TheCityUK found the impact of Brexit could range from a revenue loss of £2bn if similar regulatory regimes are introduced, to a revenue loss of £20bn if it could only rely on regulation from the World Trade Organisation.
However, the report described the financial services sector as being "sticky" and was optimistic London would be able to retain its position as a major global centre for finance.
The report also urged the government to clarify regulatory frameworks as soon as possible in order to preserve trade relationships with the EU.
"There are specific concerns for some of London's strongest sectors, and the regulatory regime that will govern their access to the European single market following Brexit.
"Sectors such as financial services are not exclusive to London, but higher value-adding functions are concentrated within the capital, and generate significant tax revenues. Access to sectors such as finance and law is also of huge importance to other EU states, so interests should be aligned to securing a supportive regulatory infrastructure."
"The government should confirm that regulatory frameworks for services will be prioritised in discussions of future trade relationships with the EU."
Several firms such as J.P. Morgan have already spoken about the possibility of moving their headquarters to Europe.
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