Councils are sacrificing school playing fields without knowing whether they are needed or not, according to the Sports Council for Wales.
Playing fields are being lost at the rate of one a day throughout the UK - but in Wales, there is no way of assessing the loss because the statistics are not collected.
In the past two years, the Sports Council - which has to be informed of any proposals affecting fields with pitches - has opposed 24 developments, but 14 have gone ahead and three are still pending.
Dr Huw Jones, chief executive, said councils should undertake a supply and demand analysis before getting rid of playing fields.
"Councils need to question the needs of a community. It's not good enough just to build when you don't know the answer to the question," he said.
The Assembly government does not keep a record of school and other playing fields, although it plans to review its guidelines on collecting data at a meeting later this summer.
Earlier this year, it published an action plan for play that compels local authorities to provide for children's play needs.
In England, the National Playing Fields Association says 45 per cent of playing areas have been lost in the past decade.
Rhodri Edwards, of the Association's Wales branch, said: "I can't tell you how many fields are under threat - we're pushing the Assembly government to keep a record.
"The Sports Council for Wales is consulted about playing fields that have a full pitch, but if it's a smaller piece of land for younger children we have no knowledge of a sale.
"Sport England is consulted about all school playing fields and other open spaces. The Assembly ministers for education and sports don't believe the problem is acute. We would say, how do you know without the statistics?"
John Watson, of children's charity Barnado's, is also campaigning for safer open spaces: "Playing fields have been lost at a rate of one a day in the past eight years, and there are now 80 acres of golf course for every acre of children's play space."
Some politicians want to get rid of the "jumpers for goalposts" type of games and invest in modern sporting facilities. But that is no excuse for disposing of open spaces, says Lynne Hayes, a Wrexham housewife who launched Save Open Spaces Wales after fighting various sell-offs in the north Wales town.
"Not all kids want to train for the Olympics, most just want to use their play areas to have fun without grown-ups," she said.
The campaign has growing support among Assembly members of all parties, and 17 have now signed a "statement of opinion" demanding that more is done to protect school playing fields.
Lynne Hayes's campaign started on her doorstep in Wrexham four years ago.
She has since been involved in fighting to save seven playing fields in the borough, with mixed results.
"Playing fields are being nibbled away at the edges, if they carry on like this we will be left with fields the size of postage stamps," she said.
She is annoyed that protection in Wales seems to be lower than in England:
"Politicians are paying lip service to the idea of community facilities. We need to make it watertight."