Jamie Dimon says the cryptocurrency bitcoin is a “fraud,” and if any of his traders dealt in it they would be sacked for sheer stupidity. Given that he runs JPMorgan Chase, which makes its money from shuffling traditional currencies around, one might suspect Mr Dimon of talking his own book. Is he?

Financial authorities increasingly agree with him. China has banned companies from issuing their own token currencies, and is considering proscribing cryptocurrency exchanges. On Tuesday, the UK’s Financial Conduct Authority issued a terse warning on initial coin offerings, which followed a similar comment from Hong Kong’s market regulator.

Mr Dimon is clearly correct in one sense: there is an irrational speculative bubble in cryptocurrency prices. The value of the best-known digital currency, bitcoin, has risen eightfold in the past year. A lesser known currency, Ripple, saw its notional value increase from $500m at the start of the year to $35bn, before plunging to $19bn.