Rio Olympic relay swimmer and 2009 world champion Junya Koga has lodged an appeal with the Court of Arbitration for Sport over his four-year ban for failed drug tests, his lawyer said Friday.

An appeal was filed with the CAS on behalf of the 31-year-old Japanese swimmer on Oct. 31, his lawyer said.

Koga twice tested positive for banned muscle-building agents in March. He said at a hearing before an impartial panel of the International Swimming Federation in late August that the test result was caused by dietary supplements, and denied intentional use of banned substances.

The panel rejected his plea for a less severe sanction.

Koga, the gold medalist in the men's 100-meter backstroke at the world championships in 2009, missed out on a berth at the 2012 London Olympics but competed in the 4x100 freestyle relay in Rio.

Japanese swimmer Junya Koga denies taking banned substances