The Assembly government will face a "rash of applications" for foundation status from schools threatened with closure after giving the go-ahead to Wales's biggest secondary, according to a teaching union.
Education minister Leighton Andrews - who is opposed to further expansion of foundation schools - has awarded the status to Whitchurch High in Cardiff, allowing it to opt out from the city council's reorganisation of schools.
Mr Andrews plans to introduce legislation "shortly" to stop schools converting to foundation status - which makes them independent of local authority control and gives them control of their own admissions and finances.
But he has agreed to Whitchurch becoming a foundation school from this September, prompting anger from unions and warnings that more applications will follow.
Rex Phillips, Wales organiser of the NASUWT, said: "Any school that's under threat of closure could apply for foundation status. I think he will face a rash of applications now. On several occasions he proudly proclaimed that there will be no more foundation schools in Wales. It seems those statements were premature."
The NASUWT claims that foundation status is the first step towards the "privatisation" of schools.
But Brian Lightman, general secretary of the Association of School and College Leaders, said: "In Wales there's no real benefit in being a foundation school because there's no financial incentive. It does mean you are a little bit more independent of the local authority, but it's not like the situation in England where schools are becoming academies."
Mr Lightman, former head of St Cyres foundation school in Penarth, said: "It is more important to have a discussion about how our future school system should be shaped rather than legislate to stop people making decisions.
"If heads feel that's best for their school I don't see why they should be stopped by legislation."
St Cyres became one of only 12 foundation schools in Wales - eight secondaries and four primaries - when grant-maintained status ended in 1998.
The status of foundation schools was further complicated last week by a High Court ruling that the Assembly government acted correctly in delegating powers over the future of Brynmawr foundation school to its local authority, Blaenau Gwent.
The council wants to shut Brynmawr's 200-pupil sixth-form as part of a reorganisation, but the school argued that its status put it outside council control. The school's governors have vowed to fight on.
Last October, Mr Andrews announced his intention to bring in legislation to prevent schools from changing their status and to stop more foundation schools being established.
He said: "Each of these schools administers its own admissions, adding to the complexity of the local authority task of planning and providing school places."
Original headline: Minister is warned he has triggered `rash' of foundation applications