"For most banks, having equivalence won't prevent banks from relocating their operations".

"Banks might hope for the best but have to plan for the worst", he wrote, revealing that most worldwide banks would now have project teams in place for determining which operations needed to be moved out to ensure continuity in serving customers.

But Mr. Browne said the current "public and political debate at the moment is taking us in the wrong direction".

Germany firms alone would face £3.4 billion in tariff costs if the relationship between the European Union and United Kingdom fell back on World Trade Organisation (WTO) most-favoured nation rules. "So I think there are definite benefits for us in economic terms". The EU is a market of 500 million people with a combined economy of $19 trillion. This allows banks and insurance companies to sell their services anywhere in the single market without having to establish a base in every country in Europe. This is known as "passporting". These include contributing to the European Union budget and allowing free movement of goods, services and, crucially, workers - something U.K. Prime Minister Theresa May has said categorically the country is not willing to do.

After divorcing the EU, Britain's banking sector could be crippled by additional trade tariffs and the invalidation of the legal rights of banks providing services, said Browne.

Simon Ainsworth, Senior Vice President as ratings agency Moody's, said that he believes banks would be able to get by without passporting rights by making use of these "third-party equivalence provisions...[which]...may provide firms with an alternative means of accessing the single market". He also stressed that it is the responsibility of all European countries to ensure no barrier would be put up across the English Channel.

She also anxious about the effect of Brexit on the British economy in the hour-long session before the City bankers.

The financial industry body, TheCityUK, has claimed that up to 70,000 financial jobs could be lost if Britain.

In an hour-long session held two months before the vote, Ms May also revealed she was convinced Britain is more secure inside the EU because of the European Arrest Warrant and intelligence sharing between police and other agencies.

Where will the banks go? "Delegations from Frankfurt, Paris, Dublin, and Madrid are all coming to the U.K.to pitch to bankers". "You built London over decades, you have the infrastructure, the depth of expertise that is unmatched".

"As far as companies are concerned, Brexit has already happened".