Catholic secondary schools have been told to stop discriminating against pupils who do not select them as their first choice secondary.
Charles Clarke, the Education Secretary, has reprimanded four Catholic schools in Essex over their admissions policies. He said governors were discriminating against prospective pupils by only accepting children who had listed a Catholic school as their first choice on application forms.
The schools said children who make them their second or third choice are not truly committed to a Catholic education.
But Mr Clarke said the policy, recommended to around 490 primary and secondary schools by three Catholic dioceses in the South-east, must be stopped.
He said: "This test of Catholicism is potentially not a fair one and may act to fetter parental preference."
The ruling, which was made following an initial complaint by Essex council council, could have significant bearing on the 2,075 Catholic schools in the country.
It could also affect the Church of England, although a spokesman said he was not aware of any schools exercising such a policy.
Governors of individual church schools can set their own admissions policies, but are reprimanded if they discriminate against pupils. New national guidelines prevent them from interviewing candidates.
The London Oratory, the Catholic secondary favoured by Tony Blair for his older children, has been criticised for interviewing prospective pupils.
Mr Clarke's judgement on the four Essex schools, St Benedict's college in Colchester, St Mark's West Essex, St John Payne and Brentwood Ursuline convent schools, was criticised by heads and the clergy, who said it would erode Roman Catholic traditions.
Brentwood, Southwark and Westminster dioceses have recommended the "first preference" rule in guidance to all local schools. Father George Stokes, director of education for Brentwood, said: "We are in the business of providing Catholic schools for those who want a Catholic education, not parents who could not get their children into a local grammar school and are using us as a safety net. This will set a dangerous precedent."
Alan Whelan, principal of St Benedict's, said: "It shows a clear misunderstanding of Catholic education by the Secretary of State. This severely undermines Catholic schools."
Oona Stannard, director of the Catholic Education Service, said governors should retain the right to "seek evidence" of children's commitment to a Catholic education before they are enrolled.
The National Union of Teachers, the National Secular Society and the British Humanist Association said the cases highlighted the danger of allowing church schools to draw up their own rules on admissions. They want one central admissions policy, binding on all church schools, and taken out of the hands of individual governors.
* In another ruling on admissions to church schools, an Anglican primary in London has been told to stop giving priority for places to the children of local clergy.
The children of vicars at St James' church, Haringey were previously guaranteed a place at the neighbouring successful St James' primary. An official report says governors wanted to favour the children because they would enhance the "religious commitment" of the school.
But Charles Clarke, who has the final say on admissions rows involving church schools, said the rule discriminated against other children and should be abolished.
The ruling followed a complaint from council officials. The school said it had used the rule for years without antagonising local parents.