A Mental Health Foundation study revealed concerns that children's long-term emotional development may be harmed by the growing emphasis on classroom over playground time in schools.
June McKerrow, director of the foundation, said the report had identified fears among parents that children lost out when schools cut break times in a bid to provide more timetable time.
This led to pupils missing out on the opportunity to develop important social skills through play. Schools needed to concentrate on emotional, as well as academic, literacy.
Ms McKerrow said: "It is quite clear that children can build a resilience to developing mental health problems by building up their social skills and learning to get along with one another, and play is a very important part of building up those skills.
"The less opportunity that children have to do that, the more potentially damaging it is."
Some 56 per cent of primary schools and 44 per cent of secondary schools changed the length of their breaks between 1990-91 and 1995-96, according to research published last year by Dr Peter Blatchford, of the London Institute of Education.
Ms McKerrow said parents were also concerned about the Government's standards drive on pupils in an "already competitive world". Parents often felt they had no option but to put pressure on their children to achieve.