Tokyo - Former swimming world champion Junya Koga has been kicked out of the Japan squad for this summer's Asian Games after failing a drug test, officials said on Wednesday.

The 30-year-old, who won the 100m backstroke world title in 2009, tested positive for traces of banned muscle-building substances, according to the Japan Swimming Federation (JSF).

Koga denied taking the substances knowingly but admitted taking a diet of supplements which could have been to blame. He now faces a maximum ban of four years.

"I feel ashamed, miserable and frustrated," the swimmer told a news conference.

"I couldn't believe it - I was in shock, I thought it was some mistake," added Koga, who failed a test conducted by the World Anti-Doping Agency (WADA) in March.

"I didn't know if it was real, or a dream. I never took the substance detected intentionally. But I'm responsible for proving that. To those who support me, I'm deeply sorry."

Documents provided by the JSF state that Koga returned traces of androgen receptor modulators LGD-4033 and SARM S-22 in an out-of-competition test in Tokyo.

The swimmer has asked WADA to conduct a hearing to allow him to explain the circumstances surrounding his case.  

Koga, who won a silver medal in the 50m backstroke at last year's world championships in Budapest, has won gold medals at the last three Asian Games dating back to 2006 in Doha.

But he will now miss this year's competition in Jakarta, which begins on August 18.

The bombshell came on the eve of the Japan Open swimming meet at Tatsumi, the venue for the 2020 Tokyo Olympics aquatic events.

"Under these kinds of circumstances I can't think that far ahead," said Koga when asked about the Tokyo Games. 

"I need to concentrate on making sure I solve the issue at hand first."

Incidents of doping are relatively rare in Japan, although two cases earlier this year caused embarrassment to Olympic officials gearing up to host the 2020 Games.

In January, Japan's anti-doping body suspended top canoeist Yasuhiro Suzuki for eight years after he spiked a rival's drink with a banned substance to improve his own chances of selection for the Tokyo Olympics.

A month later, short-track speed skater Kei Saito was booted out of the Pyeongchang Olympics after testing positive for acetazolamide, a banned diuretic which is considered a masking agent.