USA Track & Field (USATF) asked Justin Gatlin to withdraw from the national championships because of his initial positive drugs test, a source close to the situation told Reuters late on Friday.

But the source said the world and Olympic champion decided to go ahead and run in the June event at which he won the 100 metres title.

Under current US rules, USATF had no authority to stop Gatlin running until his B sample tested positive in July.

Gatlin's agent Renaldo Nehemiah declined to comment. USATF chief executive officer Craig Masback could not be reached.

Gatlin announced on July 29 he had tested positive for testosterone or its precursors following a relay race in Kansas in April. He said he did not know how he had failed the test and that had never knowingly taken banned substances.

The joint 100 metres world record holder faces a life ban unless cleared since this would be his second positive test.