LaShawn Merritt of the US reacts after winning the men's 400m final during the Beijing Olympic Games in this August 21, 2008 file photo. [Photo/Agencies]

RALEIGH, North Carolina - US Olympic and world 400 metres champion LaShawn Merritt has been suspended through July 2011 after testing positive for a banned substance found in a male enhancement product, the US Anti-Doping Agency (USADA) said on Monday.

The Beijing Olympic and 2009 world gold medallist had three samples test positive for prohibited testosterone prohormones, the anti-doping agency said in a statement.

He was suspended for 21 months, beginning October 28, 2009, the date of his first sample.

Merritt had accepted a provisional suspension in April "as a result of positive drug tests caused by his use of an over-the-counter male enhancement product," the American's attorney, Howard Jacobs, said in a statement at the time.

The statement said Merritt had used the over-the-counter product, which contained DHEA and pregnenolone, following the 2009 season.

Merritt acknowledged again on Monday he had made a mistake and said he was appreciative the three-member arbitration panel "agreed that I have never taken any substance to gain an advantage on the track," he said in a statement.

"I look forward to returning to competition as soon as possible and proving that my prior successes, including at the Olympic Games, were achieved honestly and fairly."