SEOUL, South Korea » South Korea’s former Olympic swim champion Park Tae-hwan has failed a recent doping test, his agency said Tuesday.

Team GMP said in a statement that Park tested positive for a substance banned by the World Anti-Doping Agency but it didn’t immediately disclose what substance it was and when the swimmer took the test.

The 25-year-old has been a national swimming hero in South Korea since he won a gold medal in the 400 meters freestyle at the 2008 Beijing Olympics. It was South Korea’s first Olympic medal in any swimming event and no other South Korean swimmer has since won an Olympic swimming medal. Park claimed two silvers in the 400- and 200-meter freestyle events in the 2012 London Olympics.

"Park Tae-hwan is more shocked by the results of a doping test than anyone else," the Team GMP statement said.

Team GMP officials blamed the results on an injection administered to the swimmer by a local hospital, which offered him free chiropractic and other treatments two months before last year’s Asian Games held in the South Korean city of Incheon. Park won one silver and five bronze medals at the Asian Games and he passed several doping tests during the event, according to Team GMP and swimming officials.

Team GMP officials said the hospital had earlier told them the injection didn’t contain any prohibited substance, but it was later found that it had a banned substance. Team GMP said it’s preparing to take a legal action against the hospital to find why it gave Park the injection.

The Seoul-based Korea Swimming Federation says it is discussing Park’s case and that it has no further official comment.