BEIJING (AFP) - China's Olympic swimming star Sun Yang failed a doping test in May and was subsequently banned for three months, China's official Xinhua news agency reported Monday.

The ban, following a positive test for the stimulant trimetazidine, was imposed in July, the agency said, citing the China Anti-Doping Agency.

Sun competed in the Asian Games in September, where he took gold in the 1,500 and 400 metres freestyle, and the 4 x 100 relay.

It was not known why the positive result had only come to light now, or how Sun was able to take part in the Incheon games.

It said that Sun tested positive on May 17 during China's national swimming championships, waived his right to have his B sample tested, and defended himself in a hearing in July "where the experts decided to hand him a three-month ban".

Trimetazidine was added to the World Anti-Doping Agency's banned list in January this year. Sun said he used it for medical reasons and had been unaware that it was included on the list, it added.

Sun won the 400 and 1500 metres freestyle events at the 2012 London Olympics, and also has five world championship golds to his name, but has frequently faced controversy during his career.