Adam Gemili was left baffled after he was disqualified in the 100 metres final at the Diamond League on Sunday.
The 23-year-old -- part of the 4x100m relay squad which became world champions last week -- false started and was given a red card at the Alexander Stadium in Birmingham.
CJ Ujah went on to win in 10.08 seconds ahead of an all-British field with James Dasaolu and Zharnel Hughes second and third respectively.
Gemili will run in the next Diamond League meet in Zurich Thursday.
He told the BBC: "Honestly, I don't know [what happened]. I didn't feel like I false started. It's the first time in my career that's ever happened to me. I know I'm in great shape, but I haven't had an individual race since the World Championships to prove it.
"This was a great chance -- the crowd is here and the track is amazing. I just feel like I've let so many people down. It's just absolutely gutting. I don't have any words to say. I know it's only the Grand Prix -- it's not the World Champs, but it feels the same for me.
"This was my redemption race to show everyone that I am in good shape and back running fast. I just want to say sorry."
Ujah also became a relay world champion with Gemili in London last week and is targeting a sub-10 second race in Zurich with his personal best 9.96 seconds.
He said: "I'd be happy with around 10 seconds flat, or under 10 seconds, I think that's what's going to win it. But I can't look for times, when the field is fast you have to focus on winning. "Zurich is key, I've had a consistent season in the Diamond League so I'd like to end on a high. I'm going to try to cap it with a win."
Jake Wightman won the men's mile, beating Chris O'Hare into second, having only made the semifinal of the 1500m at the World Championships this month.
But Nethaneel Mitchell-Blake and Danny Talbot, the other 4x100m relay world champions, came fifth and sixth in the 200m.
Dina Asher-Smith, who came fourth in the 200m and won a 4x100m silver, finished fifth in the women's 100m, which was won by Olympic 100m and 200m champion Elaine Thompson.
Holly Bradshaw was second in the pole vault, Kyle Langford was sixth in the 800m and Andrew Pozzi was sixth in the 110m hurdles, behind winner and world record holder Aries Merritt.
Jonnie Peacock won the T44 100m in 11.03s while Lynsey Sharp came second in the women's 800m.
World high jump champion Mutaz Essa Barshim won with a leap of 2.40 metres -- a world lead this year -- and declared he was taking the bar back to Qatar. He said: "I'll pay, I don't care, I'm taking it home."