By Robert Pigott
BBC Religious Affairs Correspondent
The UK has the fifth largest Jewish population in the world
The UK's Jewish population is growing for the first time since World War II, research suggests.
The rise appears to be due to a growth in the size of ultra-orthodox Jewish families, according to researchers at the University of Manchester.
The population fell to a historic low of 275,000 in 2005, but that figure has since increased to 280,000 in 2008.
The figures were based on UK census data and the monitoring of Jewish births by academics.
The Jewish population of the UK decreased by 40% from just after World War II to a low point in 2005 as the birth-rate among secular Jews declined and more married outside the community.
Britain has the fifth largest Jewish population in the world.
Dr Yaakov Wise, of Manchester University's Centre for Jewish Studies, says the population has now risen, to about 280,000, and attributes the growth to the extraordinary fertility of strictly orthodox families.
Given the increase in the size of the UK's population, the decrease in that of the Jewish minority was even more marked according to Dr Wise.
At the start of World War I there were half a million Jews in Britain, but in a smaller overall population.
Dr Wise calculates the proportion of Jews in the UK now is only about a quarter of its peak.
Dr Wise - who says his research is based on regular monitoring of Jewish births - attributes the decline in the Jewish population to the fact that about half of more secular Jews marry outside the community, and many of them do not bring their children up as Jewish.
He says secular Jewish women - coming from a relatively well-educated and prosperous section of society - have had on average only 1.65 children.
The UK average is 1.8.
However, the very high birth-rate of the minority of strictly orthodox - also referred to as ultra-orthodox - Jewish families is having an increasing impact on the population as a whole.
They marry young - often in their early twenties or even late teens - and have an average of almost seven children.
It means that elderly people in the community can have hundreds of descendents.
Although other researchers have made more conservative estimates of growth among strictly orthodox Jews, Dr Wise says almost three of every four Jewish babies are now being born to them.
He calculates that at current trends strictly orthodox Jews will outnumber their more secular counterparts by the middle of the century.
Referring to the reversal of the long decline in Britain's Jewish population as a whole, Dr Wise says "the birth-rate has exceeded the mortality rate for the first time since the war in each year since 2005."
The growth is particularly noticeable in cities such as London and Manchester.
Dr Wise said: "In Greater Manchester, for example, the ultra-orthodox number over 8,500 which is almost a third of the 28,000 Jews in the region.
"This is up from around one quarter only ten years ago."
"Approximately half of all the Jewish under-fives in Greater Manchester are ultra-orthodox.
"And in Greater London the ultra-orthodox now account for 18% of the Jewish population, up from less than 10% in the early 1990s," he added.
The strictly orthodox population is quite heavily concentrated in these cities, with families living and working in relatively narrow areas.
Dr Wise says the ancestors of many of these strictly orthodox families came to Britain since World War II, as the result of events such as the Hungarian uprising in 1956 and the Soviet invasion of Czechoslovakia in 1968.