India has barred one of the world's largest quant trading firms, Jane Street, from accessing its securities market after an investigation found it made "unlawful gains", taking the most stringent action ever against a foreign trading firm.

The lawsuit alleged that the traders had stolen a “ highly valuable, unique, and proprietary ” trading strategy, which was quickly revealed to revolve around India’s booming options market. Jane Street and Millennium settled the lawsuit in December, but judging from SEBI’s report it has cast a long (and expensive) shadow.

The Jane Street controversy has prompted the Securities and Exchange Board of India (Sebi) and stock exchanges to intensify scrutiny of the trading strategies employed by global high-frequency trading (HFT) and quantitative (quant) firms.