When Huishan Dairy’s shares fell 90 per cent without warning last month, it was a heart-in-the-mouth moment for shareholders in the Hong Kong-listed company. That applies especially to its chairman, Yang Kai: he was witnessing the evaporation of $3.1bn of loan collateral, putting his control of the group in doubt.

While the fall was spectacular, taking less than 45 minutes, it was only the latest in a series of similar collapses to hit the city’s equity market. Together, they raise questions about undisclosed borrowing by big shareholders who use their stakes as collateral.

In Huishan’s case, Mr Yang had pledged 71 per cent of the company’s shares to obtain loans. Only one loan, secured with 26 per cent of its share capital, had been disclosed before the shares plummeted.