Look at Britain's Jews from the outside and you will see a shining success story. An influx of poverty-stricken refugees a century ago has evolved into a middle-class community with superb educational facilities, vibrant cultural life and outstanding achievement in many fields.

However, from the inside, it looks very different, say Keith Kahn-Harris and Ben Gidley. The two academics have written a book entitled Turbulent Times, an analysis of the Jewish community over the past 20 years. British Jews, they claim, are worried about their shrinking numbers, are riven by religious divisions and by a growing rift over Israel, and are scared by what is perceived as a new and virulent form of antisemitism. But Kahn-Harris and Gidley do not paint a gloomy picture of the community today. On the contrary, they argue that the fears and insecurities faced by Jews over the past 20 years or so have encouraged a period of renewal and re-assessment which has produced a far stronger, more vital community.

Gidley explains that an important part of the change the community has undergone emanated from a belated acceptance of Britain as a multicultural society. He explains: "Our community developed in a mono-cultural society in which we were told to identify as loyal, white British subjects. This model was so successful that we were lagging behind other communities in adapting to multiculturalism, and didn't really get a place at the multicultural table until much later. Finally, in the 1990s, this did happen. "

This process of Jews acquiring self-confidence and expressing pride in their identity resulted in a cultural flowering which Kahn-Harris attributes in great part to the contribution of the Chief Rabbi Lord Jonathan Sacks. "It might be seen as controversial to say this in certain quarters, but we are relatively positively inclined towards Jonathan Sacks, although the office of the Chief Rabbi cannot be seen as anything other than anachronistic. In the 1990s he raised an agenda which had to be raised and he raised it in very public and almost a brave way. He was not the only one responsible for the changes that occurred but he was a major factor in turning around the supertanker."

This agenda manifested itself in concern about assimilation and inter-marriage and provoked the formation of Jewish Continuity, an initiative aimed at answering the question: "Will we have Jewish grandchildren?" as well as the concept of renewal of the community.

There is an irony, says, Kahn-Harris, in the fact that these initiatives led inadvertently to the growth of anti-establishment and subversive movements such as Jewdas. "The Chief Rabbi and Jewdas are connected. Even if both abhor each other, they are both connected to a more general trend that the Chief Rabbi set in motion. It shows that sometimes things can have unintended results. Even if the Chief Rabbi had in mind the idea of halachically Jewish people doing 100 per cent halachically Jewish things, his renewal set in train something much wider."

Kahn-Harris and Gidley argue that the cultural flowering of new institutions, movements and organisations, the most striking of these being Limmud, might have been partly in response to the Chief Rabbi's initiative but evolved separately from the community hierarchy which was slower to adapt. Says Kahn-Harris: "Neither the Chief Rabbinate, the United Synagogue nor the Board of Deputies has changed radically in the past 20 years but the community is more vibrant. This shows what is possible within the existing structures. But it also shows how much creativity has gone into bypassing these structures - not perhaps with the intention of destroying them. Limmud certainly doesn't intend to replace the Chief Rabbinate or the Board, but rather to bypass them and create a kind of reverse takeover."

So are the official community structures becoming irrelevant amid this political and cultural flowering? Gidley thinks not. "Within all communities there are centrifugal tendencies of creativity on the margins and there are also centralising tendencies. Without the community organisations there would be no community and without marginal voices like that of Jewdas there would be stagnation."

One of the other significant changes in the community dates from the beginning of the 21st century with the identification of a new kind of antisemitism coming from Islamist groups. Kahn-Harris claims: "Unlike inter-marriage and assimilation, the problem of antisemitism is hugely controversial. It feeds into the issue of Israel which is increasingly a source of discord. There is among mainstream organisations an acceptance that there is something called the new antisemitism but this is not a consensus shared across the community."

However Gidley is convinced that this new form is something we should all fear. "The numbers are clear. Antisemitic incidents are up and its nature has changed - the old-fashioned enemies of the far right have been replaced by a new threat. However, although we are right to take precautions to protect ourselves, it needs to be dealt with in a measured way."

While the religious divide between the various denominations remain as wide as ever, the real rift in the community comes from elswehere, says Kahn Harris. "Since the Stanmore Accords in the early 1990s, there is not the same outward conflict. Everyone knows where they stand. People have bypassed these divisions and tried to make them irrelevant." The same cannot be said of the issue of Israel, where pro-Palestinian movements such as Independent Jewish Voices have provoked serious hostility from those who have maintained their support for Israel.

However, the future, say the pair, is bright as long as the community continues to tolerate discussion between its various wings. As Kahn-Harris says: "We should be celebrating our diversity, and be open to this cultural renaissance. Let's hope the vibrancy of contemporary British Jewry is not a flash in the pan."

'Turbulent Times' is published by Continuum at £19.99