The Diocese of Westminster wants to appoint its own director of education, Paul Barber, onto the governing body in a move seen as an attempt to bring the comprehensive into line following a series of series of hostile disagreements.
The sought-after school, which has four applications for every place, is regularly rated at the top of league tables and sends more pupils to Oxford and Cambridge than almost any other state comprehensive.
But last year the relationship between the school and the Catholic diocese hit a new low in a row over admissions.
Church leaders shopped the school to the admissions watchdog amid claims that its entry rules were too elitist and effectively penalised the less devout. It was ordered to change its admissions policy for 11-year-olds after being told that parts of it were "unlawful" and “unfair”.
Last month, the diocese, which can appoint up to 11 of the 20-strong governing body, effectively removed three governors in favour of four new appointees.
As the term started at the school this month, it was disclosed that Mr Barber – one of the key players in the admissions row – was among the new governors appointed to the school.
But on Tuesday a group of parent governors was granted an interim injunction by the Administrative Court, blocking the new appointments for at least seven days. They are now applying for a permanent injunction.
News of the unprecedented legal challenge – financed by the school’s parents’ association – was circulated to families of all 900 pupils on Tuesday.
The letter says Mr Barber’s appointment would represent a “conflict of interest” because of his links to the diocese. It is also claimed that the diocese is acting “illegally” because none of the governors appointed by the church are parents of children at the school.
Headmaster Michael Gormally retired in the summer and it is understood the row is preventing the appointment of a permanent successor. Other key decisions affecting the school could also be put on hold until governors' meetings, which have been suspended, are re-established.
One governor told the Telegraph: “The management of the school is arguably hamstrung by all this.
“It is clearly not good for morale, not least because the headmaster has just retired through sickness and the school needs to get on with appointing his successor.
“The clear aim behind this is to bring in people who will not rock the boat with the diocese. Why interfere with a school that has such an excellent track record?”
The letter, signed by John Murphy, one of the parent governors, says: "I appreciate that it is unusual and unsettling for such legal action to be taken. However... we find ourselves in a situation where we do not believe that the governing body is properly constituted."
Catherine Utley, who has a 16-year-old daughter at the school, said: “Anyone who wants to improve education in this country should be watching comprehensive schools like Cardinal Vaughan.
“Why on earth opt instead to interfere and change the way so successful a school is run?
“The diocese thinks that it can somehow improve other schools by making schools like Cardinal Vaughan less good. But the way to improve failing schools is not to destroy successful ones.”
In a statement, the Diocese of Westminster said: “Paul Barber is one of four new foundation governors who have been appointed to the governing body of the Cardinal Vaughan Memorial School following vacancies that arose at the end of August.
“Foundation governors are appointed in the name of the Archbishop and form the majority of the governing body of a Catholic school.
“As with all foundation governors, they have a legal duty to preserve and develop the Catholic character of the school and to ensure it is conducted in accordance with its trust deed.
“The diocese is satisfied that all the new appointments have been properly and lawfully made.”