China's top backstroke swimmer Ouyang Kunpeng has been banned for life for a doping offence just 42 days before the start of the Beijing Olympics.

The Chinese Swimming Association (CSA) confirmed on Friday Ouyang had tested positive for an illegal substance, dealing the Olympic host nation an embarrassing blow.

Chinese swimmers were embroiled in a series of doping scandals in the 1990s but the country's sporting authorities had pledged to weed out drug cheats before the Beijing Games.

The 25-year-old Ouyang won three individual silver medals at the 2006 Asian Games and three golds at the 2003 world university games.

A CSA spokesman confirmed the positive test and said an official notice had been posted on the website of the official China Sports Daily paper.

"The swimmer Ouyang Kunpeng tested positive in an out-of-competition test on May 1," it read.

"The Chinese Swimming Association decided to give him a life ban ... although we have done lots of work, this positive case still happened, which is a deep lesson for us.

"We are going to take a clear stand on anti-doping work and firmly crack down on any violations."

His coach Feng Shangbao has also been banned for life, the notice said.

There was no immediate confirmation of what substance Ouyang had tested positive for, although Japan's Daily Yomiuri quoted an unnamed Chinese Olympic Committee (COC) official as saying it was an anabolic steroid. The COC were unavailable for comment.

Ouyang is the biggest-name Chinese swimmer to fail a dope test for several years following the country's sudden rise to swimming superpower status -- one that was clouded by suspicions of systematic doping.

In the most notorious case, four Chinese swimmers failed pre-competition doping tests for the diuretic triamterene before 1998 world championships in Perth.

Another female swimmer, Yuan Yuan, and her coach, Zhou Zhewen, were disqualified from the championships after being caught with 13 vials of the muscle-building human growth hormone at Sydney airport.