Lord Sebastian Coe said he would feel "uncomfortable" if sprinter Dwain Chambers ever competed at the Olympics.
Coe (right) would be not be happy if Chambers is selected for Beijing
Chambers has served a two-year drugs ban but has been permitted to take part in the world indoor trials on Sunday.
"Would I be comfortable seeing Dwain Chambers in an Olympic vest? The answer is no," said the chairman of the London 2012 organising committee.
The British Olympic Committee said it would oppose any attempt by Chambers to compete in Beijing.
Under a BOA bye-law, athletes who have served doping suspensions are automatically barred from competing in the Olympics. This rule has yet to be legally challenged.
Chambers said at the weekend: "I may be the first. I'd love to compete in Beijing, any athlete would."
However, BOA chairman Lord Colin Moynihan said: "There are four points. First, he is ineligible. Second, he is eligible to appeal. Third, he has launched no appeal to date. Fourth, we would vigorously defend any legal challenge to the anti-doping bye-law."
The BOA are currently conducting a review of doping procedures and the issue of the bye-law will be considered, but this will not be completed until 2009.
Although world 400m champion Christine Ohuruogu saw her Olympic suspension lifted late last year, her ban was for missing three out-of-competition tests.
Coe added: "We have to accept that the sport has been in fragile times. These have been a very difficult four or five years for us."
Meanwhile, UKA chiefs said they did not have "sufficiently strong legal grounds to refuse Chambers an invitation" to the trials in Sheffield.
The athletics body wanted to prevent Chambers from running because he had not undergone a drugs test since November 2006.
UKA took Chambers off the UK Sport's drug-testing list because they thought he had retired from the sport after the Londoner opted to pursue a career in American football.
Chambers will run in the 60m this weekend and has already met the event's qualifying standard for the world indoor championships next month in Valencia, having clocked a time of 6.60 seconds in Birmingham last Sunday.
The 29-year-old will need to win this weekend's Sheffield race to be guaranteed a spot at the world championships.
If he finishes second then it will be up to the UK selectors whether to include him in the team.
Chambers is currently ranked British number three over 60m behind Simeon Williamson (6.60) and Craig Pickering (6.57).