US Olympic and world 400 meters 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.

International Olympic Committee Director of Communications Mark Adams said yesterday that Merritt will not be able to defend his title at the 2012 London Olympics because of the ban.

"The (IOC) rule states very clearly that any athlete sanctioned for six months or more will be banned from participation in the next edition of the Games.

"This rule is still valid and applies to all athletes," Adams said.

"Each case is of course different and subject to potential legal procedures but the IOC's rules are clear, coming into force just before Beijing in July 2008."

In its verdict the American Arbitration Association said the IOC's rule went against the World Anti-Doping Agency code and, as a signatory to the code, the IOC could not ban Merritt from the London Games.