Blessing Okagbare - a favourite for the women's 100m title in Tokyo - has been dramatically ruled out of the competition after returning a failed drugs test.
The 32-year-old Nigerian sprinter finished fastest in her heat on Friday and was due to compete in the semi-final on Saturday evening ahead of the final several hours later.
In a statement, the Athletics Integrity Unit confirmed that Okagbare has been suspended after one of her recent samples tested positive for a banned human growth hormone.
"The AIU collected the sample from Ms. Okagbare during an out-of-competition test on 19 July," it said. "The WADA-accredited laboratory that analysed the sample notified the AIU of the adverse analytical finding at mid-day Central European Time yesterday, Friday 30 July.
"The athlete was notified of the adverse analytical finding and of her provisional suspension this morning in Tokyo. She was scheduled to participate in the semi-finals of the women's 100m this evening.
"The AIU will make no further comment on this matter at this time."
Team GB has three athletes, including Dina Asher-Smith, in the semi-finals of the event from 11.15am today, with the final following on at 1.50pm.