A leading Jewish state school was cleared yesterday of racially discriminating against the son of a convert in a ruling that shores up the whole faith school system.

JFS, formerly the Jewish Free School, refused a place to an 11-year-old boy whose mother had converted to the faith.

His father took legal action on the ground that the school's admissions code breached race laws, by favouring children with Jewish-born mothers over religiously observant families who had converted. A High Court judge decided, however, that the school had not discriminated on racial grounds and said that Jewish status could not be determined by secular courts.

Mr Justice Mumby recognised that, if the case had succeeded, it would probably have rendered unlawful “the admission arrangements in a very large number of faith schools, of many denominations”. He said that members of a religion did not necessarily have to practise that faith.

Judaism is passed on through the maternal line, or through conversion. Religious state schools are allowed to use faith-based criteria to decide which children to admit.

The father of the boy, known as M, was seeking a judicial review because he said that the school in northwest London used ethnic rather than religious reasons for refusing his son a place. Children from two other families who consider themselves Jewish have also been turned away from the school, which achieves high results and is very oversubscribed.

One was the daughter of the school's head of English, Kate Lightman, whose husband David is an Orthodox Jew. She converted more than 20 years ago under Israel's Chief Rabbi, but her daughter was not deemed to be Jewish by the Office of the Chief Rabbi in Britain, which controls JFS's admissions.

Dinah Rose, QC, representing M, said that the school would accept a child of Jewish-born “committed atheists” but exclude others who are “Jewish by belief and practice” because of their mother's descent.

Rejecting the legal challenge, the judge said the admissions policy was “not materially different from that which gives preference in admission to a Muslim school to those who were born Muslim, or preference in admission to a Catholic school to those who have been baptised”. He said such policies were a “proportionate and lawful means of achieving a legitimate end”.

The judge said the school had the right to give preference for those from a certain religion, even if they had fallen away from that faith. Russell Kett, chairman of governors at JFS, said: “The school abhors all forms of discrimination and welcomes the judge's express finding that JFS does not racially discriminate.”

Simon Hochhauser, president of the United Synagogue, said: “We are pleased that JFS's admissions procedures and policies have been so fully endorsed. We acknowledge the judge's ruling that Jewish status can only be defined by Jewish law.”

Philip Hunter, the chief schools adjudicator, ordered JFS last year to scrap admissions criteria designed to be used if it were ever undersubscribed.