American sprinter Tyson Gay has tested positive for a banned substance and has pulled out of next month's World Championships.


Gay, the 2007 100m world champion, said he was notified by the US Anti-Doping Agency (USADA) on Friday that his A sample from an out-of-competition test in May had returned a positive.


His B sample is yet to be tested.


Gay, 30, ran the fastest 100m time of 2013 to win the US World Championships trials in June.


He ran 9.75 seconds, matching his wind-assisted semi-final time in Iowa, to equal the 10th fastest 100m in history.


That was followed up by clocking the fastest 200m time of the year with 19.74 seconds .


More to follow.