Great Britain's David Florence was knocked out of the C1 canoe singles as France's Tony Estanguet lifted gold.
Florence, the Beijing silver medallist, hit a gate and picked up two penalty points to crash out in the semi-finals.
The world number one, 29, was 10th with just eight going into the final but can still win a medal in the doubles with Richard Hounslow.
"David has had a fantastic year so far, getting up to number one in the world. He built up well throughout the Olympic cycle, so it's a massive shock not to see him in the final.
"But it's a tough environment at Lee Valley with the home crowd and it's something which the British athletes have never experienced before. David is not saying it was nerves, but it must have had an effect."
Estanguet was unstoppable in the final, with Sideris Tasiadis of Germany taking silver medal ahead of Michal Martikan.
Florence said: "I was not good enough today. This is the race I have wanted to perform at for the last four years, but that's canoe slalom for you.
"I don't think I made any massive mistakes, there were just little things here and there. I have a couple of days to refocus before the C2 [doubles] and I have to try and pick things up for that."
The Briton came into the event in good form, becoming the first man in history to
win two gold medals at a canoe slalom World Cup across two classes.
However, he never really found his rhythm at Lee Valley, needing a second run in the heats to qualify for the semi-finals.
On semi-finals day, the Briton started strongly, but a slight mistake put him out of position and he hit his head on gate 18. From there, it was a struggle to the finish.
Estanguet, who won Olympic gold in Sydney and Athens but missed out on the final in Beijing, produced the fastest time of the competition, knocking two-time Olympic champion Martikan off the top.
The slalom competition continues on Wednesday with Britain's Hounslow going in the men's K1 kayak semi-finals.