Debbie Dunn withdraws from Olympics after failed drugs test

American sprinter Debbie Dunn has pulled out of the London Olympic Games after a positive drugs test.

The US Anti-Doping Agency have confirmed that Dunn's first urine sample tested positive for a banned synthetic testosterone substance, but she has voluntarily withdrawn to try to clear her name.

Dunn, who was part of the 4x400m relay team for the Games, said: "While I work with USADA to resolve this matter, I am withdrawing from my relay pool position.

"I do not want any issue like this to distract from my teammates' focus for the biggest meet of their lives. I wish Team USA [the] best in London as I work toward resolving this matter."

The 34-year-old has never competed in the Olympics.