The Supreme Court ruling this past Wednesday upheld the previous Appeal Court ruling about eligibility for entry into the Jewish Free School (JFS). It not only highlights the difficulty in answering the question "Who is a Jew?" but may also have made history by giving a brand-new definitional twist of Jewishness - ie, Jewish by ethnicity.

In refusing a place at the JFS on the basis of a pupil's parental descent, the Supreme Court ruled that the school has been guilty of ethnic prejudice, which in turn contravenes UK racial laws.

The president of the Supreme Court, Lord Philips of Worth Matravers, considers the matrillineality test "a test of ethnic origin." I am certain the main denominations of Jewish religious organisations such as the Orthodox, Reform and Conservative define and consider Judaism as a religion - not of ethnic origin.

So who is a Jew? There are a variety of answers.

In biblical times it was the religion of the father that determined if one was a Hebrew, ie, patrilineal descent. For example, the sons of Joseph-Ephraim and Menasseh (the founders of two of the 12 tribes of Israel) had an Egyptian mother who was the daughter of a pagan priest. And patrilineality is still the determining factor for the alleged priestly caste - the Cohens.

After the fall of the Second Temple in AD70, during the Talmudic period, the Rabbis formalised a new definition of Jewishness based on matrilineal descent. This decision was occasioned by the terrible pillage and rape of women during the fall of the Temple, which made it impossible to determine who the fathers of the subsequent children were. It was a protection for the child. This, surely, is an unnecessary safeguard today.

The Jewish Enlightenment (Haskalah) at the beginning of the 19th century saw the rise of new Jewish religious denominations - Reform, Conservative, Liberal and so on. The definition of Jewish status widened substantially. The US Reform movement and the UK Liberal movement adopted equilineality - ie, if either your mother or father were Jews, you were a Jew.

In 1934 the Nazis decreed the "race-based or biological" Nuremberg laws. They invented another definition of who is a Jew: if one of your grandparents was Jewish, you were a "pure" Jew or a Mischling (mixed race) of the first, or second order.

This grandparent formula - in a bizarre twist - was adopted by the State of Israel for immigration under the Law of Return and citizenship. However, for the definition of Jewish (nationality and religion) by the Israeli Ministry of the Interior the traditional rabbinic definition applied. Confused? So are most Jews.

After the Second World War, as the influence of religion as a unifying element weakened worldwide, and secularism grew, yet another definition emerged. Anyone who felt that he or she was a Jew, was a Jew. This was particularly true in the world's largest Jewish population in the US and also when Israel was established in 1948.

These are the main definitions of who is a Jew today, and many of them are mutually exclusive, or unacceptable to one or the other Jewish denomination.

So who is a Jew? My answer would be that anyone who says that he or she is a Jew, is a Jew. One cannot stop people from saying that. It is not illegal anywhere as far as I know. However, there is a significant and important background or rationalisation to this statement. The origin of the Jewish people came at a time when all the people in the world were religious, albeit in different ways - polytheistic or monotheistic.

However, today there are more than one billion people throughout the world who fall into a rising single sector, ie those who have no religious belief at all - the "Nones" - although many may consider themselves culturally Jewish, Muslim or Hindu, say, they do not practice a religion or profess a belief in a deity.

If one thinks about the definitional term Judaism as culture which includes the concept of religion, then we have an easy universal description. Each group within that culture can make its own rules as to who is a Jew. Certainly matrilineality is an anachronism which has also become a small minority definition everywhere. In the US and Israel where more than 80 per cent of Jews live, the majority are secular and have discarded the anachronistic rule of matrilineality. Nor do they consider Judaism merely a religion - they look on Judaism as a culture, a term which includes religion.

In a free society religious Jews who wish to have their own schools according to their sects or denominational principles (matrilineality/equilineality etc) should do so, but they should not keep on depending on public funding as they have done up to now in the UK. If there is to be public funding one has to take into consideration demographic and social facts, including the fact that the Orthodox are a small minority among Jews (however defined) everywhere including the UK.

Percentagewise, the largest proportion of Jews worldwide are secular. Equity demands there should also be government funds for the secular Jewish majority.

The laws in the US are stricter in the sense of separation of government and religion, so that all religious schools, including Jewish day schools, are privately funded. In the US public taxpayer funding for (Jewish or Hebrew) "cultural or language oriented" Charter Schools is possible only on a strictly secular basis - that is, the school is mandated to teach the culture (but not the religion) that the language represents. In the Israeli public school system there are distinct streams for the majority secular and the minority three religious streams.

At the end of the day it is Jewish culture that remains the main possible common denominator and glue that could hold all the disparate groups of different religious views as well as the majority secular Jews together, and around which they could unite if they wanted to and still maintain their individual distinctiveness.

The author is the founder of the Posen Foundation, which examines what Judaism means as a culture.