"This sector should help shape our negotiations with the EU," Lord Mayor Andrew Parmley will say, at the Annual Bankers and Merchants Dinner at Mansion House.
"This sector's voice is of paramount importance," he is expected to say. "In order to protect its ability to create jobs and growth across the nation. To protect the 2.2m strong workforce in UK financial and related professional services. And to protect those with whom they work."
Pressure is growing on the Prime Minister to adopt a softer approach to Brexit following last week’s General Election, in which she lost her House of Commons majority.
The Lord Mayor will call on May to seek a deal with "reciprocal market access" combined with a liberal immigration policy. May had previously sought an end to freedom of movement from EU states.
"One thing made clear is that this great, global City thrives on attracting and exporting international talent and capital," he will say.
"We surely all recognise that UK immigration policy should enhance our future workforce – enabling industry to fill specialist roles, build links with academia, and share staff and knowledge around the world."
Parmley will add: "In the wake of the General Election, the cities of Westminster and London have a shared duty, to work together, to minimise uncertainty and maximise calm."
Speaking to City A.M. just days before the election, the PM said she recognised "the great strengths of the City" and vowed to "work with the City and with financial services, recognising their needs and their interests as we are going through the negotiations."
However, reports this week suggest she is at odds with her chancellor, who is pushing for a more business-friendly Brexit.
Various senior Conservative figures including former Prime Ministers John Major and David Cameron, and former party leader William Hague, have urged May to change tack during negotiations with Brussels. Talks are due to begin next week despite the UK's hung parliament.
Hammond is also due to speak at Mansion House tonight, alongside governor of the Bank of England Mark Carney. The chancellor is expected to reassure firms and infrastructure projects that currently tap up EU investment funding.