Nine out of 10 schoolchildren are not doing enough exercise to ensure healthy adult lives, a study suggests.
Only 10% of schoolchildren exercise for one hour a day
Only 10% of the 4,500 11-year-olds from Bristol whose activity was monitored got the one hour a day of physical activity recommended by the government.
The findings - part of the Avon Longitudinal Study of Parents and Children - will increase fears that pupils are becoming "couch potatoes".
Inactive lifestyles are linked to obesity and illnesses like diabetes.
Researchers from London and Bristol had monitored activity levels among the children using motion sensors called accelerometers, The Observer newspaper reported.
Some did no exercise at all, others did some, but only a handful did the recommended one hour a day.
'Kids should be doing 60 minutes a day of moderate to vigorous exercise to be truly healthy and ensure their bones and cardio-vascular systems develop properly," Professor Chris Riddoch, head of the London Sports Institute at Middlesex University, who led the research, said.
Mr Riddoch, a Government adviser on children's health and expert on physical activity, said children did a lot of walking but not enough vigorous sport like football and running.
The growing number of overweight children proved they were not burning off enough calories through exercise, he said.
Boys were much more likely than girls to do an hour a day, while children from lower socio-economic groups did more than those who are better-off, according to the study.
Neville Rigby, from the International Obesity Task Force, told The Observer that children lived in a society where parents were afraid to let them out to play, there were too few school playing fields and streets were packed with cars.
"We have to create safer spaces for them to play in so they have an incentive to leave their keyboard behind and go outside, and reduce the amount of fatty and sugary foods they consume," he added.
The Department of Health declined to comment on the findings, the newspaper said.
A spokeswoman said the Government's Physical Education School Sport and Club Links strategy was working to increase the amount of PE and sport in and out of school.