Tyson Gay of the US gestures after winning in the 100m event of the Lausanne Diamond League meeting in Lausanne, July 4, 2013. [Photo/Agencies]

London -- Former double world sprint champion Tyson Gay delivered a further body blow to his troubled sport on Sunday when he pulled out of next month's Moscow world championships after failing an out-of-competition dope test.

Tyson has run the fastest three 100 metres of the year and his clash with Jamaica's Olympic 100 and 200 champion Usain Bolt would have been the highlight of the championships.

Instead he has withdrawn from Friday's Diamond League meeting in Monaco and the world championships on the worst day of a bad week for the central sport of the Olympic Games.

Also on Sunday, former world 100 metres record holder Asafa Powell and Olympic 4x100 metres relay silver medallist Sherone Simpson said they had both tested positive for the stimulant oxilophrine at last month's Jamaican championships.

Oxilophrine has similar properties to ephedrine, although it has a different chemical structure, and both are on the World Anti-Doping Agency banned list.

Powell, 30, who has been in good form recently said he had not wilfully taken supplements or substances that broke any rules.

"I am not now, nor have I ever been, a cheat," he said in a statement.

Simpson, 28, who finished equal second in the 100 metres at the 2008 Beijing Games and won a gold medal in the 2004 Athens 4x100 metres relay, also denied knowingly taking a banned substance.

"This is a very difficult time for me," she said in a statement.

"As an athlete, I know I am responsible for whatever that goes into my body. I would not intentionally take an illegal substance of any form into my system."

Earlier sources close to Jamaican athletics said five athletes, including two Olympic medallists, had tested positive for banned performance-enhancing drugs at the championships.

The managers for Bolt and world 100 metres champion Yohan Blake said their athletes were not involved. Blake did not compete at the championships because of injury while Bolt won the 100 metres.

Jamaica, the sunlit Caribbean island which currently dominates world sprinting, was hit by another doping scandal last month when twice Olympic 200 metres gold medallist Veronica Campbell-Brown was suspended by her national federation after a positive test for a banned diuretic.