A schools watchdog has partially upheld claims that a Jewish school's admissions policy might discriminate racially against non-Orthodox Jews.
JFS said admission to the school was a religious, not a racial matter
The JFS, a state comprehensive in London, has been told to change under-subscription criteria which could result in indirect discrimination.
But the adjudicator rejected a claim against the school's main criterion, that a pupil must have a Jewish mother.
Complaints had been made by Brent Council and two parents.
The school, in north London, is ranked outstanding by Ofsted and says it is oversubscribed - with more applicants than it has places.
The chief adjudicator, Philip Hunter, said the case involved a "very difficult" issue that could only be decided finally by a court.
The central objection was that the school broke the Race Relations Act 1976 by giving priority for places to children recognised as being Jewish by the Office of the Chief Rabbi.
The chief rabbi of the United Hebrew Congregation of the Commonwealth (OCR) - the school's religious authority - says that to be Jewish, one's mother has to have been born Jewish or been through a recognised conversion, or one must be converted oneself.
Dr Hunter said the school's first criteria in its admissions policy was based on religious grounds, not those of race.
"It appears to me that the policy reflects an essentially religious view, i.e. the view that those who satisfy the requirements of Orthodox Judaic law for being Jewish should be the ones given priority for admission to the school," he said.
So the policy did not contravene the prohibition on direct discrimination on racial grounds.
But Dr Hunter objected to criteria relating to who the school would admit if it did not have enough applicants who were Jewish under Orthodox Jewish law.
The school's policy gives preference to children who have one Jewish parent or grandparent, ahead of those of other faiths or none.
He said: "I accept that there is no direct discrimination as the criteria are still based on religious grounds not racial grounds (albeit the religion of the father or the grandparents).
"However, those criteria may conflict with the prohibition on indirect racial discrimination.
"The point to bear in mind here is that the criteria deal with the way in which two children - neither of whom are Jewish in the religious sense - would be dealt with in terms of admission.
"One child (the child with a Jewish father or grandparent in the religious sense) is given preference over the child who has no such parent or grandparent.
"That does appear to me to put persons who are not ethnically Jewish at a disadvantage as compared with those who are."
The school said it was pleased that the central aspect of its admissions policy and its underlying principle had been upheld.
"It is greatly reassuring that the determination of the criteria for admission of Jewish children to JFS has been confirmed as being a religious, not a racial matter, and the authority of the office of the Chief Rabbi to determine the Jewish status of our applicants has been confirmed," a spokesman said.