The Catholic Church is threatening to pull out of St Augustine's upper school in Oxford - one of the nine joint schools in the country it runs with the Anglicans.
It wants to close the 629-pupil school, lauded by inspectors for the positive attitudes it promotes, following reorganisation of the city's first, middle and upper schools into primary and secondaries.
The fate of St Augustine's, founded 18 years ago, will be one of the first decisions for Vincent Nichols, the new Catholic Archbishop of Birmingham. It will be a difficult one for the Archbishop, chairman of the Catholic Education Service, who is already involved in a high-profile Church of England project.
He is one of two assessors on the C of E's review group, chaired by Lord Dearing, looking at the expansion of church schools.
The majority of governors and staff at St Augustine's, as well as he C of E diocese in Oxford, want to save the joint school - a move supported by the county council.
The school comes under the Catholic archdiocese in Birmingham, which, to date, has favoured closure followed by the opening of a new Catholic secondary in the city.
Ten local priests have signed a letter expressing their wholehearted support for a new Catholic secondary in Oxford, claiming it represents the wishes of the majority of parishoners.
Governors, meanwhile, said the school already offers the benefits of all Catholic schools - Mass on holy days, religious education taught by Catholics and a Catholic chaplain. Anglican pupils also have a chaplain and strong links to parishes.
The admissions criteria at the school allows for Catholic students to be admitted on a 2:1 ratio.
Canon John Hall, general secretary of the Anglican board of education, said: "These joint schools are not the lowest denominator but the highest common denominator of ecumenicism."