The Education Secretary, Michael Gove, has relaxed Government regulations on the amount of minimum outdoor space schools must provide pupils for playing team games.
Campaigners fear the move, which was approved just a week before the London 2012 Games, will jeopardise the Olympic legacy for grassroots sports by leading to the further sell-off of playing fields.
Tim Lamb, chief executive of the Sport and Recreation Alliance, called on ministers to maintain a minimum space requirement for playing fields. He said: "Without a minimum requirement, the danger is that short-term expediency could jeopardise the ability of schools to deliver sport for future generations."
The revised guidelines were approved on 19 July and will come into force at the end of October.
Previously, secondary schools were expected to provide pitches ranging from 5,000 sq m to 35,000 sq m for schools with 600 pupils or more.
But under the new rules they must simply provide "suitable" outdoor space for the teaching of physical education and for students to play on.
A Department for Education spokesman said: "Our extremely strict rules on playing fields will stay firmly in place. This Government has only approved sales if the school has closed, has merged, or if equal or better facilities are being put in their place."