M's mother converted to the Jewish faith before he was born
Jewish schools may have to change admissions rules after the Appeal Court held that ethnic tests of Jewishness amount to racial discrimination.
A London school, the JFS, rejected a boy whose mother's conversion to Judaism it did not recognise.
Faith schools may discriminate on religious grounds but the Court of Appeal held that this involved a test of ethnicity - which is unlawful.
The United Synagogue says this will have "a very serious effect".
In future schools would need to adopt a test of religious practice and guidance would be issued on this - pending a successful appeal or change in the law.
Range of pupils
The state-funded JFS, formerly the Jews' Free School, is heavily over-subscribed.
It gives preference to applicants whose "Jewish status" is confirmed by the United Synagogue - which requires that the mother be Jewish.
It has pupils from a wide range of religious and cultural backgrounds including from atheist, Catholic or Muslim families - but whose mothers are, in its terms, Jewish.
The boy - named in court only as M - has a Jewish father. His mother converted to the Jewish faith before he was born but had been a Roman Catholic.
But the conversion was not recognised because it was conducted in a Progressive not an Orthodox synagogue.
The three judges - Lords Justice Sedley and Rimer, and Lady Justice Smith - said it was clear that Jews constituted a racial group defined principally by ethnic origin and additionally by conversion.
To discriminate against a person on the ground that they were or were not Jewish was therefore to discriminate on racial grounds.
"The motive for the discrimination, whether benign or malign, theological or supremacist, makes it no less and no more unlawful."
They said: "The refusal of JFS to admit M was accordingly, in our judgment, less favourable treatment of him on racial grounds.
"This does not mean ... that no Jewish faith school can ever give preference to Jewish children. It means that, as one would expect, eligibility must depend on faith, however defined, and not on ethnicity."
The United Synagogue said the decision affected any branch of Judaism that defines who is a Jew on the basis of descent (whether matrilineal or patrilineal).
It said Jewish schools of any sort - Reform, Liberal, Masorti, Charedi, Orthodox, Federation and so on - would be prohibited from giving priority to applicants who were a member of the Jewish faith.
It added: "In future, all Jewish schools (whether state or independent) will need to adopt a religious practice test, until such time as the Court of Appeal's ruling is successfully overturned or a legislative amendment is made."
"Unless the Court of Appeal decision is overturned on appeal it will have a very serious effect on all Jewish schools and on many of our communal organisations."
So it strongly supported the decision of the governors of JFS to seek leave to appeal and was consulting its own advisers on what else might be done.