Membership of synagogues has fallen below 80,000 for the first time in 30 years as "mainstream" Jews marry out of the community, a report has found.
Despite a growing number of synagogues membership numbers have fallen by 20 per cent in a generation as more Jews become secular after marrying non-Jewish partners.
The report, by The Institute for Jewish Policy Research and the Board of Deputies of British Jews, found that declining birth rates and increasing intermarriage mean the Jewish community is shrinking.
It said: "For certain sections of the community, the age at which people are choosing to have children is increasing and the number of children being born is declining.
"The overall effect of such trends may reduce the tendency of Jewish people to join synagogues.
Intermarriage also impacts on decisions about joining a synagogue, raising the question as to whether non-Jewish partners are welcomed by communities.
"Other factors also play their part: for example, the rise of more individualistic approaches to life and sceptical attitudes about the role of organised religion in society.
"Such shifts have led to Jewishness increasingly becoming a matter of choice rather than of birth, and making Jewish identity far more ‘fluid'."
While less observant communities are getting smaller, the report also found that strict Orthodox communities, also known as haredi, are growing, in part due to high birth rates.
Data shows that there has been a 28 per cent increase in the number of Orthodox schools open over the past decade.
Strictly Orthodox membership has risen by 139 per cent, from 4,489 in 1990 to 10,712 in 2016, which is equivalent to 240 new households each year.
The increase in the overall number of places of worship is likely to be down to growth in small synagogues, many of which are in Orthodox areas, the report said.
Dr Jonathan Boyd, executive director of JPR, said the findings showed that strict Orthodox communities were the only ones showing growth.
He said: "The trends point to a future in which stricter forms of Orthodoxy will hold an increasingly prominent position, not only in synagogue membership, but in how Judaism is practised and how Judaism is seen and understood by others.”