Failing to maintain access for UK banks to the EU market after Brexit would be an act of "economic sabotage" that would ripple across Britain, London's mayor Sadiq Khan will say on Thursday.
Banks in Britain have said they are preparing to move staff and activities to mainland Europe as soon as next year unless access to the bloc's single market is maintained.
Ireland has its eyes on a number of those institutions although their ultimate fate remains unclear.
The Irish Government is thought to have set its sights on Standard Chartered and Royal Bank of Scotland in particular while earlier this year IDA Ireland, the foreign investment agency, already pitched to UK and international lenders.
Khan, a member of the Labour Party, is due to make the remarks in a speech at a dinner in the City of London financial district.
He will also say that mishandling Brexit might have the biggest impact on financial services since “Big Bang” deregulation 30 years ago.
That event helped to propel London to the top of the global financial centre league tables, a ranking that will be hard to keep without EU market access, bankers have said.
“If the proper agreements aren’t negotiated, there will be serious knock-on impacts with jobs and billions of revenues lost - something that would hit the entire country, not just London,” Khan will say.
“It’s frustrating, to say the least, that much of what we are saying seems to be falling on deaf ears.”
While Khan’s remit as mayor does not include responsibility for setting financial policy in London, he is the capital’s most high-profile advocate and one of the Labour Party’s most influential figures.
But his position jars with that of John McDonnell, Labour’s most senior finance spokesman, underlining the rifts within Labour which critics say are watering down the party’s influence on the Brexit process.
