France is already thinking about how to nab the City's best bankers after Britain voted to leave the European Union.
Prime Minister Manuel Valls said on Saturday he is thinking about how to re-jig France's tax system to attract international firms which may be thinking of leaving London.
Sadiq Khan is working to protect City businesses' passporting rights, but many banks are considering a move should they lose their right to sell financial services in Europe after Brexit.
Speaking to Le Parisien daily, Valls said: "We know that groups based in the City are planning to leave for Dublin, Amsterdam, Frankfurt and Paris. We are working on measures that could help strengthen our attractiveness. I think notably about taxation or the status of expatriates.
"To major international companies I say: 'Welcome to Paris! Come invest in France'!"
Other destinations lining up to pocket London's big banks include Dublin and Amsterdam, but a recent poll found global institutional investors think London will remain a world-leading financial centre after Brexit.
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The French president Francois Hollande has being pressing for a speedy Brexit, saying yesterday that Britain's decision "cannot be delayed or cancelled."
Speaking at the Battle of the Somme centenary ceremonies in France yesterday, Hollande said: "Being in the European Union has its advantages and I think that is what the British people are beginning to understand, what those who are tempted by the Brexit are going to reflect upon.
"But the decision has been taken - it cannot be delayed or cancelled. Now we must take the consequences."