A High Court ruling allowing the London school where Tony Blair sent his oldest children to carry on interviewing for places could jeopardise the new national admissions sytem.
David Hart, general secretary of the National Association of Head Teachers, said: "The judge has driven a coach and horses through the code of practice. His ruling clearly means that heads will be free to carry out interviews, the content of which will be beyond anybody's control."
Mr Justice Jackson told the Roman Catholic secondary that it did not have to demonstrate "slavish obedience and deference" to the code of practice which bans interviews in all but state boarding schools.
He accepted the Oratory's arguments that interviews proved effective at determining prospective students' Catholicism when other methods failed, and refused to allow an appeal on his ruling.
One in 10 church schools held interviews to assess prospective parents' and pupils' religious commitment until the code of practice was changed in 2003. The ban on interviews was due to take effect next year.
Critics said schools also used the interviews to select by social background or ability, although the London Oratory produced evidence that its intake closely matches the national average.
The Oratory challenged a ruling by the schools adjudicator that its interviews of parents and prospective pupils were not fair, objective or transparent.
It said banning the interviews threatened to undermine its ethos by preventing checks on religious commitment. The school said written statements from parents and parish priests were sometimes unreliable.
Elizabeth Passmore, schools adjudicator, ruled in October that interviews were "neither necessary nor desirable" and said the code of practice only permits them for state boarding schools.
She said it would be impossible to ensure that the communications skills or reasoning skills of parents or their children were not being taken into account.
Charles Bear, the school's barrister, said after the case that the ruling showed the code of practice was not legally binding, although schools wanting to depart from it can still face objections and adverse rulings from the adjudicator.
"The Government wants to keep the rhetoric of schools choosing their own criteria for admissions and not appearing to be the nanny state," he said.