Almost half of Britain's male Jews are marrying outside the faith renewing fears that the community will die out in the next century.

The first large-scale survey of British Jews, conducted by the Institute of Jewish Policy Research, reveals today that 44 per cent of Jewish men under the age of 40 are married or living with non-Jewish partners, not far from the 52 per cent intermarriage rate of Jews in the United States, which caused shock in 1990. Among Jewish women, 20-25 per cent marry outside the community.

Similar intermarriage figures have been used to suggest that the Jewish community will disappear through assimilation.

Download the new Independent Premium app Sharing the full story, not just the headlines

The Chief Rabbi, Dr Jonathan Sacks, has published a book titled Will we have Jewish grandchildren?, and has said that assimilation could achieve what Hitler had failed to do. A spokesman for him said the figures were "an enormous challenge to the community."

Antony Lerman, executive director of the institute, which questioned 2,100 adults, said the figures were not good news for those who wanted to maintain the Jewish community through traditional forms of identity.

He said the survey had some important messages for the chief rabbi and other policy makers. But he added: "The survey is not all bleak. One of the things that is happening is that British Jews are coming together more like an ethnic community than a religious one.

"People who are not marrying Jews or are not in synagogues are expressing their Jewishness in different ways, for instance in involvement in cultural events."

The survey suggests that even Jews who marry out or leave the synagogue retain a strong sense of Jewish identity.

The United Synagogues, the main orthodox grouping, for which the chief rabbi speaks, represents only 40.2 per cent of Britain's 300,000 Jews, according to the survey. Another 36.9 per cent have no religious affiliation; 12.5 per cent are Reform; 3.9 per cent liberal; and 3.2 per cent the ultra- orthodox Hassidim.

Outside the Orthodoxy, British Jews are markedly more liberal in their social attitudes than the population as a whole. They are politically more right-wing than the general population but when allowances are made for social class, Jews of all religious affiliations are found to be well to the left of gentiles in the same professions.

Religious observance among Jews, the survey says, is likely to be an expression of belief in Jewishness, as much as of a belief in God. Rabbi Albert Friedlander, of the Westminster Synagogue, says: "Even Jews who do not think much about religion feel they have to sign up ... To be a Christian, you have to believe things; but in order to be a Jew, you just are."

The report says Jews believe that racism has increased more over the last five years than anti-Semitism. They are sharply divided over the Middle East peace process, but 62 per cent would give up most of the West Bank for peace, compared with 8 per cent of American Jews.

Comment, page 19