Dabbling in margin trading, as the currency markets have just reminded us, is a bit like dabbling in cooking methamphetamine. Profitable, until you blow up.

The China Securities Regulatory Commission has taken note. Over the weekend, it fined 12 brokers for violations of margin lending rules. Haitong, Guotai Junan and Citic Securities were restricted from opening new margin accounts for three months. In response, mainland “A” shares in the three fell by the daily Shanghai limit of 10 per cent yesterday; the Hong Kong “H” share lines of Citic Securities and Haitong fell 16 and 18 per cent, respectively. Taking the fines to heart, the Shanghai Composite fell 8 per cent.

The moves seem aimed less at extinguishing the market rally than at protecting retail investors, who make up 80 per cent of volume on China’s stock markets. As the market has run, leveraged exposure has increased. Margin financing through regular brokerage channels now totals $180bn, more than double July levels, according to Goldman Sachs.