Catholic bishops have told their 150 grant-maintained schools they will be cut off from the Church if they refuse to return to diocesan control when the opted-out sector is abolished.
Bishops have issued an ultimatum to the schools - which include the London Oratory attended by the Prime Minister's two sons - to return to "voluntary-aided" status or risk having the links with the diocese severed .
Grant-maintained schools have until November 20 to say whether they want foundation, aided or community status. Most are set to opt for foundation status, which preserves some of the independence they gained by opting out.
But hard-hitting letters warn Catholic GM governors that a school choosing any category other than voluntary-aided will stop being Catholic by next September.
"It would no longer have the right to call itself a Catholic school, and would cease to be part of the Catholic education in this diocese."
While most GM Catholic schools are expected to choose the aided category,more than 70 previously told The TES they do not want to return to diocesan control.
They fear recrimination from the dioceses - many of whom were hostile to opting out - and fear for their financial future.
The crux of the bishops' argument centres on the the composition of a Catholic school's governing body, staffing and the content of Catholic RE lessons.
They warn that if GM schools became foundation schools the diocese would not be able to appoint more than a quarter of the governing body.
Letters sent to governors by their local bishop said: "If your school were to end up in a category other than voluntary-aided, it is unlikely that I would be able to declare that the school remained a Catholic school."
Heads and governors were shocked by the tone of the letter. Brother Francis Patterson, chairman of the Association of Catholic GM Headteachers,said:
"It is a question of the Church exercising its power to get its schools back. I think 90 per cent of the Catholic GM schools might previously have gone for voluntary-aided status - this will now sway the rest."
Privately the Church of England considered the tactics too strong but a spokesman added: "It might just do the trick for them although I don't know how they will handle it if it doesn't."
It has issued guidance to its 120 opted-out schools and is "encouraging" them to become voluntary-aided. Any that do not will still remain C of E schools.