RALEIGH, NORTH CAROLINA (REUTERS) - United States' 100 metres record holder Tyson Gay said on Sunday he had tested positive for a substance he could not identify and was pulling out of next month's world championships.

Gay, the 2007 world champion who had the year's fastest 100 metres time, said in telephone conference call 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.

The B sample is yet to be tested, said a tearful Gay.