Tokyo Olympics: US sprinter Sha'Carri Richardson misses out on relay team, won't compete in Tokyo after marijuana ban

American sprinter Sha'Carri Richardson will not be selected to the US 4x100m relay team for the Tokyo Olympics, after accepting a one-month ban for a positive cannabis test.

The 21-year-old, who streaked to victory over 100m at the Olympic trials in June, was expected to be one of the biggest draws at the upcoming Games in Japan.

Her supporters had hoped she could still compete in the relay event, after the United States Anti-Doping Agency (USADA) confirmed the suspension last week.

"We are incredibly sympathetic toward Sha'Carri Richardson's extenuating circumstances and strongly applaud her accountability, and will offer her our continued support both on and off the track," says USA Track & Field (USATF).

Richardson told NBC that she used the banned substance to cope with the death of her mother.

The women's 100m event in Tokyo starts on July 30, two days after her ban ends, but she is unable to compete in the event, as her suspension wiped out her results at the trials.

Selection to the relay team was her only remaining chance of competing at the Games. The relay coaches are able to select two athletes on top of the first four trials finishers.

"All USATF athletes are equally aware of and must adhere to the current anti-doping code, and our credibility as the national governing body would be lost, if rules were only enforced under certain circumstances," USATF says.

The organisation believes the World Anti-Doping Agency rules regarding THC, the main active ingredient in cannabis, "would be re-evaluated".

"While our heartfelt understanding lies with Sha'Carri, we must also maintain fairness for all of the athletes who attempted to realise their dreams by securing a place on the US Olympic track & field team," it says.