The treaty, which was signed in December 2015, became effective in March this year.

In the months leading up to the treaty becoming effective, the family approached several top law firms for advice on how to go about moving the money. This intensified after local bank accounts belonging to the family, their businesses and other associates were shut down.

Refusal by the firms, including ENSAfrica, Webber Wentzel and Hogan Lovells, to help saw the family approach UK and US law firms.

Sources with knowledge of the approaches told the Sunday Times the brief was on how an "anonymous but prominent South African family and their associates" could be assisted to move money from the country.

The global law firms were also approached for advice on how to counter former finance minister Pravin Gordhan's court case over why he should not assist them in having the banks reopen their accounts.

Gupta family lawyer Gert van der Merwe confirmed his clients had approached South African law firms for "commercial advice".

"I sat in on some meetings with ENSAfrica and Hogan Lovells. The advice, for example, was on possible commercial transactions. I'm involved in litigation and don't handle commercial matters."

He confirmed the Guptas also turned to UK and US law firms, "in particular for advice on the Gordhan matter".