Two days before Britain voted to leave the European Union, Shanker Singham wrote an article arguing in favour of Remain. Yet today he is busy promoting what he calls “the Brexit opportunity”.

Why did Singham, one of the world’s leading trade and competition lawyers, change his mind? I’ve come to his offices in Mayfair to find out. But also to discover more about another, even more profound conversion.

Singham works at the Legatum Institute, a think tank “promoting policies that lift people from poverty to prosperity”. On this, the hottest day of the year, we retreat to an air-conditioned library several floors up.

Dressed in a dark suit and a bright red tie, Singham sips a cup of coffee. Throughout our conversation his phone lights up with messages from MPs asking for urgent briefings. He has an air of calm busyness – an apparent contradiction that might explain how he is able to work seven-day weeks without burning out.

For the past 25 years, Singham has been involved in some of the world’s most significant economic transitions: the transformation of the Soviet Union, Poland’s entry into the EU, the Latin American apertura, US free trade negotiation and China’s accession to the World Trade Organisation. A joint US-UK citizen, he counselled Mitt Romney and Marco Rubio during their presidential runs.

Singham’s surname is Sri Lankan. His Tamil parents came independently from the former colony and met in London. His father worked as an accountant, his mother as a microbiologist. “It wasn’t the easiest transition to make and they certainly made a lot of sacrifices,” he says. He went to St Paul’s School (whose past pupils include Pepys and Milton), then Oxford.