The City's new Lord Mayor Charles Bowman will this evening make a direct appeal to Theresa May, asking her to kick-start trade talks with the European Union.
In his speech at the Lord Mayor's Banquet, which will be attended by the Prime Minister, Bowman will press the government to enter phase two of the Brexit negotiations.
Bowman is expected to say: "Negotiations should soon enter phase two, with trade talks on the table.
"I know I speak for everyone here when I say that the City will do its utmost to support you (the PM) in a task that will define our place in the world for generations.
"Just like you, we want a good deal that works for Britain, and for our European partners…and for the many other financial centres around the world with whom we do business."
Bowman will echo the calls of others in the City who want a transitional deal to give businesses time to adjust to the new trading relationship between the UK and the EU.
In addition, he will urge the government to provide certainty to all EU and UK nationals about their status if they live abroad.
His comments come after an intervention by his policy chief, Catherine McGuinness, who wrote to the chancellor last week warning that US banks fear a "disorderly Brexit".
McGuinness held a series of meetings with US business figures at the beginning of the month, and found some see a "bad Brexit" as a risk to global financial stability.
"Key actors are deeply concerned about the impacts of a disorderly or chaotic Brexit," she said. "In their eyes, this is a rapidly increasing possibility."
It is thought that banks will start triggering their contingency plans in the first quarter of next year if the government does not secure a transitional deal, which would keep the UK inside the customs union and the Single Market for a set period after March 2019.
Speaking at the City A.M. awards last week., Brexit secretary David Davis said the future of the Square Mile was a top priority in Brexit negotiations.
"When I sit round the negotiating table with Michel Barnier, or round the Cabinet table with my colleagues, it's with the importance of the City very much in my mind," he said.