Olympic canoeing champion Tim Brabants has suffered a major blow in his bid to defend his K1 1000m title at next year's London Games.
The 34-year-old has been ousted from the Great Britain team for next month's World Championships in Hungary after being beaten in a race-off by Paul Wycherley.
Brabants was forced into the race-off after a run of poor form and illness.
The result means Wycherley is more likely to compete for GB in London.
Brabants became the first Briton to win a gold medal in either the sprint or slalom kayak discipline with his success in Beijing.
He had previously won bronze medals in the 1000m and 500m at the Sydney Games in 2000 and became 1000m world champion in 2007.
Brabants, who was made an MBE in the 2009 New Year Honours List for his services to kayaking, spent 18 months away from the sport after the 2008 Games to focus on his career as a doctor.
He could yet defend his Olympic title if 25-year-old Wycherley suffers injury or loses form, with final selection trials scheduled for spring next year.
Failing that, Brabants is expected to form part of a larger two-man or four-man boat at the Games.
Wycherley, from Guildford, lost the first of the race-off contests against Brabants at Eton's Dorney Lake on Wednesday but forced a decider by winning the second.
The third and final race took place on Thursday, with Wycherley winning it by a margin of well under half a second.
"That was one of the toughest races I've done," Wycherley told BBC Sport's Ollie Williams. "Tim is the Olympic champion and an enormously brilliant paddler.
"The last half of the race was a war of attrition. I had to dig very deep - and then dig some more."
Technically, selection for the World Championships has no bearing on who will occupy the boats that Britain sends to the London Olympics.
But if Wycherley qualifies the K1 boat for the Olympics - by finishing in the top eight at the Worlds - the selection policy makes it hard to unseat him.
"It's a dream come true and where I've wanted to be my whole life but it's the first day in a very long process," said Wycherley.
"I still believe Tim is the best K1 1000m paddler in the world but my best today happened to be a bit more than his best.
"I've no doubt he's going to come back at it very hard. I don't think he's been beaten in Britain before, so I'm sure he's going to come out guns blazing.
"He's probably very disappointed not to have won that today and he's taken himself away at the moment. But he's a top bloke and next time we meet I'm sure it'll be a handshake and a slap on the back."
The 2011 World Championships will be held in the Hungarian town of Szeged from 18 to 21 August.
Brabants, who won a silver medal at last year's Worlds, could still compete in Hungary as a member of a larger boat, most probably the four-berth K4 in which he has been gaining experience this season.