South Korean prosecutors announced charges on Friday against a doctor for giving Olympic swim star Park Tae-hwan an injection without disclosing it contained the banned steroid testosterone.

The 24-year-old four-time Olympic medalist blamed the injection, administered in July, for a positive drug test on urine samples he provided in September, prior to the Asian Games.

Briefing local media, a spokesman for the prosecutors' office said the doctor, identified only by his surname Kim, was being charged with causing injury through negligence, the Yonhap news agency reported.

The news that Park had failed a doping test broke last month and his management agency filed a formal complaint that triggered a prosecutorial probe of the hospital where he received the injection.

The spokesman said Park had asked if the injection contained any substances that might show up positive in a dope test, but had been told by the doctor that there was no cause for worry.

While accepting the doctor might not have known testosterone was a banned substance, prosecutors said he had a professional obligation to confirm all the contents of any medicine he administered and their possible risks.

According to the Chosun Ilbo daily, Park received the injection as part of an "anti-aging" procedure he had been offered for free.

Dubbed "Marine Boy" in South Korea, Park became the country's first Olympic swimming medalist when he won gold in the 400 meters freestyle and silver in the 200 meters freestyle at the 2008 Games in Beijing.

He also won silver in both events at the 2012 London Olympic Games, along with 400m gold in the world championships in 2007 and 2011.

But he has struggled to repeat that form and failed to win a title at his home Asian Games last year, competing in a pool that bore his name.

His positive doping test will be the subject of a FINA hearing set for February 27.