The 27-year-old, also a key member of the teams which brought relay gold for the United States in the last Olympics and World Championships, tore a ligament in the second toe of his left foot while training in July.

"Right now my focus is to get healthy, prepare for next season, and try for my second 400m Olympic gold," he said.

The injury will not require surgery but the recovery period is expected to be as much as 12 weeks.

Wariner, the world's top-ranked 400m runner in 2010, has been plagued by minor injuries this season.

He has clocked only the 12th fastest time this year, and was beaten in the US Championships - which double as the world trials - by college athlete Tony McQuay.

Wariner won the Olympic 400m title in Athens in 2004, and followed that with World Championship wins in 2005 and 2007.

He was runner-up to fellow American LaShawn Merritt - who has just returned to the sport after serving a two-year sentence for a doping infraction - at the 2008 Olympics and 2009 World Championships.

Wariner is expected to be replaced on the US team by fourth-placed trials finisher Jamaal Torrance at the World Championships, which take place on August 27 - September 4.

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