“Be it a hard Brexit or a soft Brexit, this cannot go on after Brexit. Those who think that after Brexit things can be just as before are making a major mistake”, he added.

The UK aims to start talks with the EU on the terms of their divorce at the end of March.

The British government has said it wants to retain full access to the single market, but also to curb EU immigration.

EU leaders have said it cannot have both, with one potential casualty being the loss of “passporting rights” for London-based firms to do business in the rest of Europe.

Dublin, Frankfurt, Milan, and Paris - the EU’s other financial hubs - stand to gain if there is a banking exodus from London.

Sapin on Friday said he wants to cooperate with the City of London in future.

“Are we going to go into a war with London? Of course not”, he said.

Zdenek Turek, the CEO of Citibank Europe, a Dublin-based offshoot of a US bank with operations in 21 EU countries, said at the Tatra congress on Saturday that Brexit uncertainty is causing headaches in his sector.

“Everybody is looking at the worst case scenario because there’s no lead from the politicians now on what’s going to happen”, he said.

“Today, in every bank of a certain size, you have a department looking at what might happen if there’s a complete breakdown of passporting”, he added.

He said UK banks’ loss of access to the EU would lead to higher costs for firms and their clients.

He also said the idea that the Brexit talks would settle the issues in just two years, as planned, is unrealistic.

“You need a transition period, at least in our industry … There’s a danger of cliff effect - the old agreement is gone and the new one is not in place yet,” he said.

The Tatra event also saw a broader debate on the future of eurozone integration and related projects, such as the EU banking union, after the UK has left.

European house

German finance minister Wolfgang Schaeuble said on Friday that “our European house” is “badly in need of repair”.

He said the political climate is not right to rewrite the EU treaty to create a fiscal union, but he said eurozone integration should continue at a more pragmatic level.

“We should continue with [integration of] the banking sector, push forward on capital markets union, to stick to the fiscal rules that we have agreed”, he said.

Finance commissioner Pierre Moscovici said “I’m a federalist like him [Schaeuble], and, like him, a realist”, adding that fiscal union is “not for tomorrow”.

Ivan Korcok, Slovakia’s EU affairs minister, said euro-integration is one of the few areas where there is support from both the political elites and the general public to go further.

Citibank’s Turek said the business community would like to see further banking harmonisation, stricter EU budgetary discipline, and labour market reforms to make it easier to sack people for the sake of competitiveness.

Recalling Schaeuble’s “European house”, Turek said: “We’ll all have to move and live and that house and give up our little houses where we love today … national sovereignty will have to be further surrendered”.