A loophole in rules aimed at protecting playing fields is being exploited by property developers wanting to build on them, say the Liberal Democrats.
Special planning rules apply to playing fields
Local councils have to consult community sports body Sport England about bids to develop such land unless it has not been used for five years.
This is leading some developers and councils to fence off fields so they fall out of use, the Lib Dems claim.
Sport England raised the issue in a recent consultation on planning laws.
Public and privately owned playing fields are used by many schools which do not have such facilities on site, as well as community groups.
Lib Dem local government spokesman Tom Brake has written to ministers urging them to close the loophole so no more playing fields are lost.
He said: "How can ministers expect children to become healthier and fitter if playing fields are being sold off.
"With obesity among young people we need to ensure that all our playing fields are available for young people doing their PE or taking part in sports activities."
Although the government had made some progress in protecting playing fields, it was acting far too slowly to reverse the damage caused by previous Conservative governments, he added.
Under current legislation, local planning authorities are required to consult sports advisory body Sport England in a formal capacity about any planning applications relating to land that has been used as a playing field at any time in the previous five years.
As a statutory consultee, Sports England's comments have greater weight and they can apply greater pressure to the local authority.
And if it objects the matter is referred to the Secretary of State.
In practice, developers and councils can sidestep the rule by fencing the playing fields off, or not managing them as pitches, for the pending five year period.
The Lib Dems say this is a growing problem especially in areas like London, where the price of land means developmental pressures are most acute.
Sport England said it wanted to see extra additional protection for playing fields with the removal of the "five year rule".
This would mean they would have to be consulted on any development on a playing field, irrespective of whether it had been used for that purpose or not in the last five years.
"This is to stop potential landbanking of playing field sites by developers, whereby they are boarded up and use restricted so as to avoid Sport England being consulted and compensatory provision being provided," a spokesman added.
The Department for Communities and Local Government said strong protections existed whether or not Sport England was consulted.
"Playing fields should not be developed unless an assessment of need has shown them to be surplus to requirements.
"The government is committed to reversing the decline of playing fields and putting sport back at the heart of life in this country."