"The family philosophy was that if we made ourselves like non-Jews, we'd be accepted more," Devorah said, a few days before the traditional Orthodox ceremony. "We'd moved so far away, my father didn't even know what gefilte fish was."

Devorah, formerly a headhunter in the City, knew how easy it would have been to marry out of a faith she barely acknowledged. Instead, she has chosen a strictly Orthodox religious life, emigrating to Israel with a rabbinical scholar she met through a modern-day matchmaker. To the hardline Jewish groups battling against assimilation, Devorah's "return" marks a proud victory against the forces of modernity. In truth, it is a rare exception to a trend that is giving Britain's Jewish leaders serious reason to kvetch.

Intermarriage is on the rise. Unless that changes, community leaders fear, Anglo-Jewry could be imperilling its future. It is now 11 years since the chief rabbi, Dr Jonathan Sacks, warned that "the Jewish people, having survived for thousands of years in the most adverse circumstances, including the Holocaust, is today threatened by intermarriage and assimilation". Sacks's warning prompted a range of cultural and educational initiatives designed to instil Jewish pride in young singles. Yet the intermarriage graph kept climbing, with around half of British Jews marrying "out".

There is an entire seam of Jewish humour feeding off the anxiety that surrounds intermarriage. In one old joke, a father of three sons becomes increasingly distraught after the first declares he is marrying a Catholic, then the second says he plans to marry a Hindu. Finally, it is the third son's turn. "Dad, I'm in love with this fantastic woman called Goldberg," the young man says. "Goldberg?" the father answers, a relieved grin spreading across his face. "Yes," says the son proudly. "I'd love you to meet Whoopi..."

Lately, the subject has prompted little humour within Anglo-Jewry's representative bodies. According to the Board of Deputies of British Jews, only a third of Jews of marriageable age are getting married in synagogues. Partly, of course, this reflects the general decline in religious ceremonial. But Professor Barry Kosmin, the director of the Institute for Jewish Policy Research, estimates from his studies that today's intermarriage rate has risen to about 50%. The impact is already being felt on the next generation. Between 1990 and 1999, the Board of Deputies reported, births notified to Jewish institutions fell by a quarter, from 3,300 to 2,500. An even starker warning has come from the demographer Sergio Della Pergola of the Hebrew University in Jerusalem. Fifty years ago, around 450,000 people in Britain identified as Jews. The figure is now nearer 300,000. The fall is not just related to intermarriage: emigration and a wider lack of faith have played their part. Yet if these trends continue, Della Pergola has written, "the UK's Jewish population will decline to 240,000 in 2020, 180,000 in 2050, and 140,000 in 2080".

"It's a crisis," says Rabbi Shaul Rosenblatt, the joint UK executive director of Aish HaTorah, one of the Orthodox groups most aggressively campaigning against assimilation. "For every Jew that comes back, I expect 50 are moving away. The strictly Orthodox will remain — those couples I know have an average of seven or eight kids. But the rest — the more liberal and secular branches of Judaism — they will die out."

Once Rosenblatt starts to point fingers, it becomes clear how far Anglo-Jewry's branches are divided over a response to assimilation. At his home in north London (his nearby office was destroyed last summer in a suspected anti-semitic arson attack; nobody has been caught), the rabbi blames rising intermarriage squarely on the Reform and Liberal synagogues, which have reinterpreted traditional Orthodox values for the modern world. "Up to 150, 200 years ago, you had no secular Jews," he says. "I don't know when the rot started, but the Enlightenment brought in this concept of humanism, telling Jews they could live secular lives. The floodgates opened. And all these hitherto-Orthodox kids just ran."

Today, Rosenblatt sees it as his mission to bring them back home. Aish HaTorah (literally "Fire of the Torah") targets secular 18- to 30-year-old Jewish singles who might otherwise be lost to the faith. Through heavily subsidised holidays to Israel or South Africa, and "cool" social events such as speed-dating nights, it presents a friendly, often sexy face designed to attract Jews who might not meet other Jews. Once the ice is broken, its approachable, usually clean-shaven rabbis will invite newcomers to Friday-night sabbath dinners or lectures on Jewish thought. Ultimately, Aish wants them to lead Orthodox religious lives and find an equally religiously committed spouse. You would not know it from its posters, which are big on images of celebrities such as Arnold Schwarzenegger, but Aish is at war with secular life. Those it rescues are known as baalei tshuva ("masters of return" but also, tellingly, "repentant sinners").

Aish has its critics. At the Coffee Cup cafe in Hampstead on a Saturday night, a favourite hangout of the north London "becks" — the secular, fashion-aware young Jews more at home clubbing than in synagogues — it stands accused of pulling families apart. "Aish has got a really bad name," says 17-year-old Flo Stein, an A-level student from Finchley. "Everyone knows them as brainwashers. One friend who got involved with them was always having arguments with his parents. His mum felt they were making him go against the family." "We're constantly being accused of brainwashing," Shaul Rosenblatt responds. "All we're doing is presenting information and letting people make a decision."

Aish played its part in Devorah Simon's "return" journey, which is why Rabbi Rosenblatt and some of his colleagues are in Finchley synagogue tonight to celebrate her marriage to Jake Greenberg, an American whose own journey took him similarly from secular life to full-time rabbinical study. As Devorah tells it, she was barely aware of her Jewish heritage and could easily have gone the other way. "We had the Christmas tree and stockings with our names on," she says. "I never had Jewish boyfriends or girlfriends." Her great-great-grandfather had helped found the Liberal Jewish Synagogue in St John's Wood, but Devorah's only remaining link was a visit during the annual high holy days.

It was a young rabbi at Edinburgh University, where she was studying history of art, who encouraged Devorah to revisit her faith. She went on to spend two years at an Aish-run seminary in Jerusalem. Becoming Orthodox has brought a new set of daily rules for Devorah, from rejecting her parents' non-kosher food to refusing to drive there on the sabbath. "My family has made a difficult journey, but when they see you as a happy, confident member of society, they take it with dignity," she says. Her mother, Doreen, accepts that the rigid rules can cause tensions: "The kosher thing can push you apart, but Devorah's always been good when she visits and she brings her own meals in tubs."

At the wedding, a few guests also seem bemused at the couple's new rule book. Wry jokes are whispered; remarks are made about the wigs (called sheitels) that the married Orthodox women wear to avoid the "immodesty" of displaying their real hair. Some guests nudge
and point when, between courses, 30 black-hatted and bearded men leave their seats to sway by the doors in prayer. The North Circular Road might be rumbling impatiently outside, but here we could be in 18th-century Warsaw.