Christine Ohuruogu is out of the 400m at the World Athletics Championships after being disqualified in the heats.
The Olympic champion, 27, who has been struggling with injury all season, was dismissed after making a false start.
Such events are extremely rare in the 400m, but Ohuruogu came out of her blocks way ahead of any of her rivals.
Ohuruogu told Channel 4: "I knew it was me straight away. I can't believe it. I just wanted to get a good start as I knew it was going to be a fast round."
Ohuruogu had been in poor form all summer after a long spell out with a quadriceps injury and had finished last in her previous race in the Diamond League in Crystal Palace.
But having banked on coming into form late in the day, just as she had in taking the world title in Osaka four years ago, her quest to regain her world crown is over before it has truly begun.
Later on Twitter Ohuruogu wrote: "Just like to say thanks for all the kind responses. Has been a tough day to cap off a tough season. I am still numb.
"I can't blame anybody but myself. And it's gut wretching [sic]. Absolutely awful.
"Don't know what to think or feel. I really did think I could turn it around this season.
"Thanks to everyone who has supported and pushed me along the way.
"I hope I haven't let you all down.
"I hope that I will find it in me to recover this and move on in the best way that I can."
Under the new false-start regulations brought in by the IAAF at the start of 2010, she was allowed no warning nor second chance and was shown a red card by officials before being ushered off the track in a state of shock.
As the heat was run she remained motionless for several minutes, clearly distressed, before walking slowly out of the stadium.
She added on Channel 4: "Of all the people it could happen to it had to be me. I'm the slowest starter, it's so ironic.
"I worked really hard to turn the last three weeks around and I am upset for my coach who worked really hard to get me in shape."
As the heat was run she remained motionless for several minutes, clearly distressed, before walking slowly out of the stadium in tears.
Ohuruogu's team-mates Lee McConnell and Nicola Sanders were more fortunate, qualifying for the second round as fastest losers after gold medal contenders Allyson Felix, Sanya Richards-Ross and Amantle Monsho all progressed comfortably.
Richards-Ross was waiting to go out for her heat as Ohuruogu was disqualified, and expressed sympathy for the woman who beat her to Olympic gold in Beijing three summers ago.
"It's so rare that you hear the second gun in the 400m - I thought it was a malfunction," she told the BBC. "In the call room we were like, aw man. I'm disappointed for her."
Ohuruogu will now seek redemption when she joins forces with Sanders and McConnell in the 4x400m relay later in the championships.
McConnell told BBC Sport: "Over the next few days we'll rally round, pick her up and get her ready for the relay team.
"We've all gone out of Championships in ways we didn't want to do and this is another way for it to happen. But she's strong, she's a strong girl and I'm sure she'll get over this and we'll do what we can to help her. I don't think there is anything you can say, just a bit of a hug and letting her know you're there."
Meanwhile, European silver medallist Michael Rimmer failed to make it through the 800m heats after finishing in fifth place, but team-mate Andrew Osagie advanced.
Rimmer has been struggling with a pelvic injury and was close to not competing in Daegu for fear of "bombing out" in the first round.
He said: "Last year was a good stepping stone and I started this season in phenomenal form. I was second behind the Olympic 1500m champion in Doha and I thought this is it now, I've barely got into training and I'm already absolutely flying.
"I thought this was going to be my year, I thought I was going to run something stupidly quick and people were going to start noticing.
"I've done everything possible in the last two months to try and sort the problem. I'm mentally absolutely battered. The last two months have been absolutely horrific."