Several investigations, including by Open Democracy and Channel 4, have revealed that Brexiteer and Bristol businessman Arron Banks used the money to fund the pro-Brexit Leave.EU campaign by raising capital from Russian investors through two diamond mines he owned in Kimberley, South Africa.

Papers filed in a civil action in the Northern Cape High Court alleged that Banks had rerouted the money from the sale of the mines to bankroll other projects, including the Leave.EU campaign.

Banks made history in making the biggest ever financial donation to a political campaign in the history of British politics. He is being investigated with regard to his alleged Russian connections and the role this played in the result of Britain’s 2016 “Brexit” referendum.

Apart from the Kimberley mines, one of Banks’ closest associates, James Pryor, is South African-born and acted as an adviser to Margaret Thatcher and FW De Klerk.

Hain, in his letter to Dick, said criminal complaints against Banks had been lodged with the Directorate for Priority Crimes Litigation in South Africa as well as the Serious Fraud Office in London.

“It has been confirmed to me that the South African authorities are taking the matter seriously and have appointed a multiskilled investigative task team,” said Hain.

He said that of concern to him was “the fact that Mr Banks has widely admitted that he led Brexit voters ‘up the garden path’ and thereby fraudulently misled them into voting to leave the EU.”

The purpose of his letter, he said, was to “formally request an investigation into the source of funds, allegedly paid by Banks to Leave.EU, to establish if same were indeed clean funds as well as to establish if there was any fraud on the voters”.

He asked the Metropolitan Police in Britain to join hands with authorities in South Africa “in case you might help them conclude the investigation from their side”.

He said that if it were found that the Leave.EU campaign had been funded with the proceeds of crime to mislead voters “then this would constitute perhaps the single largest fraud ever committed in modern British history”. DM