In the Mansion House speech that was abandoned last week due to the murder of Jo Cox MP, George Osborne was due to warn on the impact on financial services if Britain leaves the European Union.
Many major banks, such as JP Morgan and HSBC, have said that jobs could be lost in the capital.
Today, one of France's most senior economists backs that claim.
Mathilde Lemoine is a former advisor to the French Prime Minister Dominique de Villepin, and now sits on that country's equivalent of the Office for Budget Responsibility, the economic watchdog.
She told the BBC that if Brexit happens, transactions for firms across the EU in euro-denominated assets - a large and important part of the City's operations - would move to the eurozone.
It is already the European Central Bank's view that euro-denominated business worth hundreds of billions of euros would be better executed - or "cleared" - on the continent.
The Treasury disagreed, and thought it so important to the health of the City it took an ultimately successful legal case via the European Court of Justice to block the ECB's proposals.
'People can move'
Mrs Lemoine, who is now chief economist at Edmond de Rothschild private bank, said that judgement would fall if Britain left the EU.
"If the United Kingdom leaves the European Union this clearing house dealing with euro transactions will relocate to the eurozone and therefore many foreign banks, European banks and also non-European banks will relocate to the eurozone," she said.
"London is one of the best financial centres in the world due to the people, so the people can move. [The] skills will move from London to the European Union and the eurozone, and therefore they can improve the financial sector of the eurozone.
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"It's a sort of abnormality that the clearing houses dealing with euro transactions are based in the City and not within the eurozone, because it means that the ECB cannot supervise the clearing houses and clearing houses are institutions which facilitate financial transactions and therefore there's a systemic risk.
"If the United Kingdom decides to leave the European Union the eurozone member states can request immediately the relocation of the clearing houses dealing with euro transactions to the eurozone."
She says the market could be worth between €600bn and €1tn.
So, it's certainly significant and could mean thousands of jobs moving, although Mrs Lemoine said the total number was difficult to predict.
Those who support Britain leaving the EU counter that, although the UK might lose some services, freed from EU regulation on issues like pay, London would flourish.
And certainly one chief executive of a major bank told me that London was the only globally significant financial centre in Europe and was destined to remain so because of its deep, liquid markets, professional services and sheer size (Paris and Frankfurt, for example, have far smaller financial centres).
Gerard Lyons, the former economic advisor to Boris Johnson and chief economist at Standard Chartered, said that the rest of the EU would still need London.
"Whichever way one looks at it, I think London and the City will remain the major financial centre of Europe, no matter what happens in the Referendum.
"And I think the longer term outlook is to position London, not just against the EU but against future competition from Hong Kong, Singapore and New York," he said. "And I think we have a great ability to do that outside the EU."
Another fear for markets is that there could be an economic downturn following Brexit.
But Mrs Lemoine said that the negative impact might not be as brutal as many economists suggest.
"Many models predict a disaster for the UK if Brexit happens," she said. "I can understand why the models predict such a disaster, but that's not my evaluation.
"Firstly, because their prediction doesn't account for any other measures being put in place.
"It's not possible that the Prime Minister, the government and the Governor of the Bank of England would do nothing to improve the situation.
"Of course, they would act and they will implement various measures which will limit the decline in GDP. In addition, exporters can react very quickly to change in trade policies.
"Of course, all situations are unique but remember when the United Kingdom left the European monetary system in 1992? UK exporters moved from the European Union to the rest of the world," she said.
"The UK exports toward the European Union slimmed sharply, but those towards countries outside the European Union accelerated sharply.
"As a result - against all expectations - UK exports accelerated by 5% overall."