Adjudicators had objected to the admissions procedures, which included interviews, at the London Oratory school in west London, where Prime Minister Tony Blair sent his eldest children.
Following a pound;77,000 judicial review last month, the adjudicators'
decision - which came in response to objections from a local primary school - was overturned.
If the adjudicators' office is given leave to appeal and succeeds, it believes it can address the judge's concern that schools may need to interview to assess a parent's faith.
Mr Justice Jackson said interviews had proved to be an effective way to test the religious commitment of parents and pupils, while statements from parish priests were sometimes "unreliable".
He said the Catholic secondary school did not have to show "slavish obedience or deference" to the admissions code of practice, which banned interviewing at day schools from this year.
The school has been rapidly carrying out interviews in order to complete its admissions process by the deadline of January 24. But a successful appeal could force them to start again. John McIntosh, headteacher, said that would derail the new admissions system for London and the South- east, where 41 education authorities are now working jointly.
"I don't see how we could do it this far down the road," he said.
Critics said the judge's decision put the entire national admissions system in jeopardy, by allowing schools to pick and choose which parts of the code of practice they wanted to follow.
The decision could have made it possible for one in 10 church schools, which previously held interviews, to restore them against the wishes of Church leaders.
But both the Anglican and Catholic leadership backed the ban, fearing accusations that interviews discriminated against working-class pupils.
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