Norway's Eirik Veras Larsen won gold in the kayak sprint on Wednesday as Tim Brabants, defending his Olympic crown, finished eighth.
World champion Adam van Koeverden finished second to claim silver, while Max Hoff from Germany won the bronze.
Brabants, who came in to the Games struggling for full fitness following a serious tendon injury, could never contest the lead at Eton Dorney.
Drawn in lane one nearest the grandstand, the 35-year-old had the support of the home crowd but fell well behind Canada's Van Koeverden as he surged in to an early lead.
But with 250 metres to go in the individual 1000 metre kayak sprint, Larsen edged in to first place to claim gold.
Brabants said he felt good in his warm up but was disappointed with the result.
He said: "Unfortunately today the other guys were better. Everybody in that final has been in a final this year, at the world cup and the Europeans, except me.
"To be in the final was my main aim and then to try and make that podium again. I felt like I could. In the warm up I felt strong but unfortunately I felt like I was never in the race for some reason.
"The conditions weren't bad. I am not making excuses at all. I would rather not have had lane one but it didn't make a difference on the day.
"The right guys won and they showed their class, they showed their form. My main problem is letting down the home crowd. I wanted to add to the medal tally for Team GB like all the other athletes and especially like the returning gold medallists from Beijing who have done so."
"I am really looking forward to seeing our 200 metre boys racing - maybe they will pick up when I missed out."
Asked whether he intended to continue, the accident and emergency doctor said he would return to his day job, probably at Nottingham's Queen's Medical Centre, in the next few weeks but seemed to rule out competing in Rio in four years.
He added: "I don't want to give up just yet. We work on four year cycles. This is the end of one cycle and so in a few weeks time we will reassess where we are. I love the sport. I love being in this environment, so I certainly don't want to stop just yet.
"Another four years is a tough one. I will be 39 in Rio so I think it is unlikely and unrealistic but I don't want to give up just yet - there are all sorts of other competitions we can compete in."
He added: "I am happy for the guys who have won. I know they are good athletes and I know they are clean athletes, which is the key thing for me - knowing that the top guys on the podium are clean. I am happy for them but I am disappointed for me.
"There always are (athletes who dope) unfortunately but time will tell but there are guys who I have raced throughout the last 12 years who disappear for two years on a ban and then they come back and they are racing again. It's part of our sport unfortunately and all we can do as British athletes is promote drug free sport.
"As I say, I am pleased for the guys on top of the podium because I know they are good athletes and worthy champions."
