The Archdiocese of Westminster is querying whether the school, run by the Oratory Order, is flouting Government guidelines that insist admissions should be based on clear and objective criteria.
The grant-maintained school in Hammersmith, west London, takes pupils from as far away as Surrey, Hertfordshire and Kent and interviews of parents and children are a decisive part of the selection process. Euan Blair, son of the Labour leader, lives eight miles from the school.
Around 400 parents are being interviewed for the 180 places available in September - the Blair interview has already taken place - and successful candidates will be informed next month. Around 15-20 appeals against rejection are heard, but none are usually upheld.
Unlike most popular Catholic comprehensives, the school does not choose between Catholic pupils on the basis of closeness to the school. According to the school, the interview is an important and decisive part of the admission procedure. The criticism within the Archdiocese, (the Oratory is sited in the diocese, but it does not come within its remit), appears to be that the school's criteria are too vague and it is not clear on what basis children are selected.
The Catholic schools service in Westminster has already advised its primaries that they should not provide information on pupils' records to the Oratory before children have been given a place.
The Oratory's prospectus says the headteacher normally makes arrangements for all applicants to be interviewed and the main function of the interview is "to assess whether the aims, attitudes, values and expectation of the parents and the boy are in harmony with those of the school". Heads of junior schools, it says, may be asked for information about ability, effort, attendance, punctuality, extra-curricular activities and whether the applicant is suited to the Oratory.
Government guidelines issued in July last year recommend that church schools should only interview to judge religious suitability. Schools, it says, should make clear the purpose of interviews in order not to be vulnerable to criticism that judgments are based on social, ethnic or academic considerations.
The London Oratory was one of the first schools to become grant-maintained. (Its head, John McIntosh is reputed to have had the idea of schools opting out.) The former Inner London Education Authority had attempted to discourage the Oratory from using interviews as a basis for selection, but its arrangements were approved by the DFE when the school opted out.
Mr McIntosh argues that the school was not set up to serve a particular neighbourhood and the lack criteria based on distance from the school is not unfair. Applicants, he says, are assessed on five-point scale on the basis of the interview.
The DFE does not appear to have reacted to complaints from the Archdiocese - made before the row over the Blairs' choice of school. The guidelines say that in the case of GM schools, the Secretary of State has powers to act where schools are considered to be denying parents their rights as set out in the guidelines.
However, the DFE may examine the Oratory's admissions policy when it considers a proposal from the school to admit 20 seven-year-olds on the basis of musical ability.
Schools that have opted out more recently have had to comply with the guidelines on admissions. The Cardinal Vaughan, another GM Catholic comprehensive in London, sets pupils a written test in order to achieve a mixed-ability intake. Where there are too many applicants for a particular band, it takes account of whether they have a sibling at the school and the distance they live from the school. This year, 71 per cent of fifth-formers achieved five or more GCSEs grades A to C, compared with the Oratory's 54 per cent.