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This could be due to inflammation caused by excessive exercise which has been linked to poorer physical and mental health in previous studies, they said.

They analysed the teenagers' health using a scoring system developed by the World Health Organisation to measure physical and mental wellbeing together, which runs from 0 to 25 with a score of below 13 indicating poor wellbeing. The average score among the group was 17.

The teenagers were then grouped according to how much sport they played in an average week. Some 35 per cent were categorised as "low" (less than 3.5 hours per week), 41.5 per cent were "average" (3.6 to 10.5 hours), 18.5 per cent were "high" (10.6 to 17.5 hours) and five per cent were "very high" (more than 17.5 hours).

Results published in the Archives of Disease in Childhood journal showed that those in the low and very high groups were more than twice as likely to score below 13 on the wellbeing scale.

Participants in the high group had a better outcome, however, with a 50 per cent lower chance of scoring below 13 than the average group.

The best scored were among those who got 14 hours of exercise each week, but the benefits were cancelled out after 17.5 hours, researchers reported.

They said their findings could be of "major clinical interest" to doctors who advise teenagers on how exercise can benefit their health.

Although the results do not necessarily prove that excessive exercise causes lower wellbeing, results from biological studies suggest that very high leverls of exercise can cause inflammation which in turn harms physical and mental health.

"We found that the sport practice apparently ceased to be a protective factor and became an independent risk factor for poor well-being when practicing more than twice the seven recommended hours per week," they said.

"These results highlight the importance for physicians, caring for adolescents, to follow-up their level of sport practice and concurrently inquire about their well-being."