A second straight BMX Olympic final left Shanaze Reade in despair as the three-time world champion finished sixth at the London 2012 Velopark.
There was further disappointment in the men's final as fellow cyclist Liam Phillips came home eighth after unclipping from his pedal in mid-air before crashing on the penultimate corner.
Prime Minister David Cameron, his wife Samantha, Chancellor George Osborne and David Beckham were among the 6,000 crowd willing Reade to victory, but the 23-year-old Crewe rider, who crashed out of the 2008 Olympic final attempting a gold-medal-or-bust manoeuvre, could not recover after a poor start.
Reade told BBC Sport: "I'm pretty emotionless. I'm sure it will sink in, but the Olympic Games is the biggest platform you can race at and you've just got to give it your best.
"I just tried to stay focused and do my absolute best and today it just wasn't good enough."
Colombia's Mariana Pajon won, with Sarah Walker of New Zealand second and Laura Smulders of Holland third. World champion Magalie Pottier of France was seventh.
Reade's form appeared promising in the three-run semi-finals, but the one-run final, in less than 40 seconds, left no room for error and she was never in contention for the top place on the podium after a hesitant start.
Reade added: "I've worked for four years for this and it's over. I've just got to pick myself up and come back. I did everything I could. It's fantastic everyone's come out and supported me. I'm just gutted I didn't get a medal.
"I didn't want to go too early on the start and mess up my chances.
"I backed back a little bit and by the time I did that, I was on the inside, so everybody was closing in and I couldn't come back from that."