Cutting school breaktimes is short-sighted and ignores the fact that children do better if they have a chance to let off steam rather than constantly working, Play England has warned.
The charity is alarmed that despite children spending much of their waking hours at school, they are getting less chance to play.
The organisation has published a report with evidence that breaktimes are getting shorter and children's play is becoming more structured. Research has previously suggested that children's playtimes may have decreased by as much as 50 per cent since the 1970s, and it is believed that schools feel under pressure to cut back on breaks.
It is calling for the Government to publish guidance setting out a minimum allocation of time for breaktimes and lunchtimes.
In addition, a poll carried out by the charity found that half of parents say there are not enough places for children to play safely where they live, rising to 67 per cent of low-income families. Almost three-quarters of children said that school was the main chance they had to play with their friends, but 55 per cent had to bolt their lunch down in order to make time to play.
Lisa Davis, a policy officer at Play England, said: "One of the reasons schools are so important for children's ability to play outside is that parents have a fear of children playing outside where they live because of traffic and perceived stranger danger.
"Research shows that breaktimes are being shortened and afternoon breaks are being cut because of pressure on the school day to deliver the educational requirements."
Structured play, in which children's play is used as a way of teaching certain knowledge or understanding, is beneficial. But Play England points out that children need time to chose what they want to do, with no management from adults.
Miss Davis said: "We would like schools to provide spaces for physical activities, quiet activities and creative activities. Play is beneficial for healthy development and wellbeing, but it's also important for its own sake."
Andrew Lynham, head of Wellesley Primary in Yate, South Gloucestershire, has overseen a #163;60,000 revamp of the school's playing field with the help of Michael Follett, play adviser for South Gloucestershire Council. The school now has a wooden village, a bridge and tunnel, a house on the hill, play platforms, a giant sandpit and a shed of scrap that includes items to dress up in.
Mr Lynham said: "What we found was all those little niggles stopped and children would play very happily, children who struggled with making friends were getting on well, and that groups of children who didn't know what to do and would hang around the coat-tails of the lunchtime staff were now off playing on the field."
Last year, lunchtime was extended to a full hour, and now Mr Lynham is looking at how the grounds can be used more during lesson times.