However, the former world junior champion, has not been banned from athletics because cannabis is not regarded as a performance-enhancing drug. The finding comes after the suspensions for anabolic steroids of Linford Christie, Dougie Walker and Dwain Chambers, all top British runners, during the past 15 years.

Under the rules of the British Olympic Association, Lewis-Francis is now automatically barred from competing at any future Games but he can appeal and it is understood that he will be successful, since the offence is minor. Lewis-Francis has been seen as a future star ever since he won the world junior title and last year ran the final leg of Britain’s triumphant 4x100 metres relay team in the Olympics.

Lewis-Francis, 22, from Walsall, was found positive on March 5 at this year’s European Indoor Championships in Madrid when he finished second in the 60 metres sprint behind Jason Gardener, another member of the victorious Athens quartet. Lewis-Francis has been stripped of the silver medal and given a public warning by UK Athletics, the national governing body, because small traces of cannabis were found in his urine.

Lewis-Francis said yesterday: “I do not smoke cannabis. My only explanation is that I may, without realising it, have been in the presence of people who were smoking cannabis and that I passively inhaled their smoke. I have not knowingly taken this substance and have not attained any performance-enhancing benefits.”

A spokesman for UK Sport, which oversees drug policy in Britain, yesterday accepted Lewis-Francis’s explanation but reiterated its attitude to the drug. “The use of marijuana may be damaging to the image of sport and, as elite competitors can be influential role models for young people, the use of an illegal substance also sends inappropriate messages to them.” Lewis-Francis has been told that if he tests positive for marijuana again he could be suspended for two years.

Nick Davies, the media director of the International Association of Athletic Federations (IAAF), said: “An athlete testing positive for cannabis should be subject to a sanction that can range from just a public warning and disqualification from the event to a maximum of two years. The IAAF considers every case individually and it is up to the athlete to demonstrate that the substance was not taken to enhance performance.”

The IAAF is satisfied that this was true with Lewis-Francis. Mr Davies pointed out that cannabis had never been on the IAAF banned list but when, in 2003, the world governing body signed up to the World Anti-Doping Agency, which is backed by the International Olympic Committee, it was obliged to follow its rules. This has cannabis listed under the sub-section of “specified substances”, when action can be taken.

English football has been hit by several positive findings for marijuana in recent years, particularly among young players. The Football Association usually gives the player counselling rather than a suspension, unless the offence is repeated. Lewis-Francis will not receive counselling because his explanation has been accepted.

UK Sport said that Lewis-Francis will not have his lottery funding suspended.

Cannabis is banned in some sports because small amounts can cause a feeling of relaxation and reduce inhibitions.

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