JPMorgan Chase boss Jamie Dimon, the son of a stockbroker and as steeped in financial market knowledge as any executive on Wall Street, has seen more than a few of his forecasts fall flat recently. The culprit has generally been what the late Donald Rumsfeld famously described as “unknown unknowns”.

There is a higher risk of a serious fall in US stocks than is currently being reflected in the market, the head of JP Morgan has told the BBC.