The Shanghai stock exchange has said it will impose “self-disciplinary actions” on four brokerages after a series of late-day trades executed earlier this month caused “abnormal trading” in the country’s notoriously volatile equity markets.

The move relates to unusual share price movements on December 20, when a flurry of orders were placed in the last three minutes of the trading session by the four brokers, including China International Capital Corporation (CICC) and UBS Securities, a joint venture part-owned by Swiss bank UBS.

Those orders – most of which were to sell a stock – affected 17 companies, all of which are members of the FTSE A50 index popular with index-tracking exchange traded funds. Some of those stocks, including China Construction Bank and Great Wall Motor, suffered sharp share price falls.