This is the destructive reality that comes with a hard Brexit

A month before the June 23 referendum on European Union membership, a banking source told me of an unlikely, but potentially devastating, "Doomsday Scenario."

It went like this: the Leave campaign would win the referendum, David Cameron would resign, the resulting leadership battle would sweep a hard-line Brexiteer, such as Liam Fox, into Number 10, who would then leave the European Union immediately without any trade agreements.

While Doomsday did not happen, for Remainers and London's finance industry, the current discussions about a "hard Brexit" come close.

Some politicians want the UK to leave the EU as soon as possible, making it more likely that the UK will lose membership of the single market.

By scrapping free movement of EU nationals and abandoning membership of the single market, the UK risks cutting one of its most important international trade links and losing its financial passport.

Here is what is going on: