School pupils are being allowed to stay up until the early hours of the morning playing games that are inappropriate for their age, said Mary Bousted, general secretary of the Association of Teachers and Lecturers.
She said many parents were failing to adhere to age-restrictions on the most violent games, raising concerns that children are growing up desensitised to aggression and bloodshed.
It was also claimed that over-exposure to screen-based entertainment was robbing children of valuable time interacting with friends or playing outdoors – harming their education and long term development.
The comments were made ahead of the union’s annual conference in Manchester next week when members are expected to call for the introduction of “stringent legislation” on the games industry.
It follows repeated concerns from psychologists that watching violent films and playing games such as Grand Theft Auto, Mortal Kombat and Modern Warfare makes youngsters more prone to violence. Speaking yesterday, Dr Bousted said: "I think what we are talking about, first of all, is the amount of time children spend locked in their room. The fact that children spend hours locked in their rooms playing computer games, which means they're not interacting, they're not playing and not taking exercise."
Some of these games were "very violent", she said, and risk having a major effect on "tender young minds of children and young people."
Dr Bousted said that many teachers fear parents are ignoring age restrictions on computer games, which often ban their sale to children aged below 18.
"The watershed tends to work quite well, but with online TV and video children and young people are probably watching inappropriate content over a range of media,” she said.
"It's about reminding parents and carers that they have a very real responsibility for their children and that schools can't do it alone.
"It takes the very serious and labour-intensive business of proper care and attention of young children before they go to school and while at school to allow them to learn most effectively.
"If they're up to 12 or one o'clock playing computer games, and coming to school exhausted, not interacting with other children, that's not good preparation for school, and not good preparation for life."
A motion to be debated at the ATL conference says there are "negative effects from some computer games on the very young", adding that the union should commission evidence "which will enable it to lobby the government for the introduction of stringent legislation with regard to computer games".
The comments came as a study commissioned by the British Toy and Hobby Association found that 28.2 per cent of parents believe their child gets 30 minutes or less of physical activity after school.
The report – based on a survey of 1,000 parents – revealed that just one-in-20 believe their child gets more than two hours of exercise outside school, while 14.5 per cent say they get between an hour and an hour-and-a-half.