The London Oratory was accused by the Office of the Schools Adjudicator of "discriminating against pupils on their ethnicity and socio-economic background".
The schools adjudicator launched an inquiry into the boys' school in south west London after a complaint by the British Humanist Association.
Governors at the school will now challenge the accusations at the High Court.
Judges will decide whether the school, which taught the sons of former prime minister Tony Blair, Liberal Democrat leader Nick Clegg and deputy Labour leader Harriet Harman, has to revise its admissions criteria.
The report accused the school of discriminating against non-Catholics by "not allowing for the admission of children of no faith" and having an admissions policy that made it harder for students from a certain social background to get a place.
The OSA found 105 aspects of the school admissions code were broken over a two year period.
Last summer, when the report was published, headmaster David McFadden said: "The OSA has made four determinations against this school in the past six years, the most recent of which was again challenged successfully.
"Today the adjudicator's office has now, it seems, suddenly found a further 105 aspects of our admission arrangements which apparently breach the school admission code.
"The school governors once again reserve the right to refer this determination to judicial review."
The High Court hearing is expected to last a few days.