The Chinese Swimming Association (CSA) said on Tuesday that they believe in Sun Yang's innocence although the star swimmer failed a doping test in May and had consequently been banned for three months.

The multi-Olympic and world champion tested positive for stimulant trimetazidine on May 17 during the national championships.

CSA said in a statement which was handed out to a dozen of Chinese reporters waiting at the CSA building on Tuesday morning that Sun proved with sufficient evidence in July's hearing that he was not aware of the substance was prohibited and took the prescription drug Vasorel to treat a heart condition.

"Sun took the drug without an intention to improve his result, but both he and the Zhejiang swimming association (which Sun belongs to) had made mistakes," said the statement.

Sun's 1,500m freestyle title at the national championships was stripped and he was also fined 5,000 RMB (about 816 US dollars). The Zhejiang swimming association and some related officials were also punished by CSA.

"We believe Sun did not intend to cheat," said Xu Qi, team leader of the Chinese national swimming team. "Sun Yang is an excellent swimmer and his good results came from his talent and hard training.

"Sun cherishes his reputation and he had no reason to cheat in the national championships."

Xu also said that the punishment for Sun was reasonable.

"The punishment was in accordance with related rules and reasonable," he said.

According to China Anti-Doping Agency (CHINADA), both Sun and Zhejiang team doctor Ba Zhen failed to file the therapeutic use exemption when Sun took the test. Sun had been taking Vasorel from time to time since 2008 when he started to suffer heart palpitations. Trimetazidine is able to increase coronary flow reserve and was added to the prohibited list in January 2014.

Sun took to social media on Monday saying he didn't intend to cheat.

"I have been tested numerous times during years of training and competition and I had never failed once before," Sun said on his Weibo account.

"I was shocked and depressed at that time, but at the same time it made me cherish my sporting life even more. I will take it as a lesson and be more careful in the future."

Xu admitted Sun had been under a lot of pressure since the positive test.

"He has to deal with the pressure," said Xu. "And we should understand his situation and give him some help."

The three-month suspension ended on Aug. 17, before Sun's participation in the Incheon Asian Games in September, where he won three gold medals.