Ruja Ignatova and her brother Konstantin led a global campaign promoting OneCoin

It was the cryptocurrency that promised to make its earliest backers enormously rich and the world a better place by “empowering” millions of people in Africa and Asia. Now, with its glamorous Oxford-educated founder missing after amassing more than £3.5 billion from investors seeking to buy into the next bitcoin, her brother has admitted it was all an elaborate Ponzi scheme.

Konstantin Ignatov, the Bulgarian-German brother of Ruja Ignatova, 39, pleaded guilty to charges including money laundering and fraud in a deal with prosecutors. If he complies with the terms, he will no longer face criminal charges beyond those for tax offences.

He implied in testimony in New York that, having quit his job at a dog-shelter in Germany to work on OneCoin, he was…