A few months ago a group of more than 100 people dipped beneath the waters of a river in Madagascar. They had not gone simply to bathe; they were performing ritual immersion to complete the requirements of their conversion to Judaism.

They may have just inaugurated one of the world's newest Jewish communities but some of the Malagasies who took the religious plunge believe their connection to Judaism is much older. They view themselves as descendants of the ten lost tribes, who were scattered after the Babylonian conquest of the kingdom of Israel around 2,700 years ago.

They are by no means an isolated phenomenon. In various pockets of Africa and Asia, different groups have begun to identify with Judaism and establish contact with mainstream Jewry. Even if there is scant evidence for the claim of lost tribe descent, the myth continues to exert a powerful imaginative hold.

Not all groups trace their origins to ancient Israel. Beit Yeshourun, on its website, describes itself as "a warm and welcoming Jewish community" in the town of Sa'a in central Cameroon, an hour's drive from the capital of Yaounde.

Formerly a Christian community, their study of the Bible led them to explore Judaism and in 1998 they resolved "to serve God by the same way Jews serve Him". Their leader Serge Etele had been due to come to Britain to speak about his community at the Limmud conference but, unfortunately, was thwarted by entry rules which have twice denied him a visa to Britain.

The myth of the lost tribes exerts a powerful imaginative hold

A similar story to Beit Yeshourun is that of the Abayaduya in Uganda, now estimated to be more 2,000 in number and shortly to celebrate their centenary. They owe their genesis to Semei Kakungula, a military leader converted to Christianity by British missionaries who circumcised himself and adopted Judaism.

Some groups may actually have Jewish roots, if not from the biblical Israelites but from Jewish traders who came to their region hundreds of years ago and settled.

Whatever the explanation, their existence has begun to be taken more seriously by the rest of the Jewish world. Not only are organisations such as Kulanu in the United States and Shavei Israel in Jerusalem helping these Jewish outposts to get recognition, but last year Israel's Ministry of Diaspora Affairs set up a committee to investigate the whole issue.

One group has already been embraced by Israel, the Bnei Menashe from north-east India, who emerged in the 1950s. While the 7,000-strong community claim ancestry from the descendants of lost tribes and therefore see their practice of Judaism as a return rather than a new departure, it is possible they are another example of a group originally converted to Christianity who eventually opted for the earlier monotheism.

Although they have not officially been recognised as zerah Yisrael, "seed of Israel", the former Sephardi Chief Rabbi Shlomo Amar was sufficiently impressed with their dedication to support their immigration. Some 3,000 Bnei Menashe have moved to Israel, where their integration is being helped by Shavei Israel; around 700 are due to arrive this year. But there has been controversy over the fact that many have been settled on the West Bank.

More recently, in the late 80s, another group has surfaced in southern India, the Bene Ephraim (Ephraim was the brother of Joseph's son Menashe). They opened their first synagogue in Kothareddypalem in 1991.

While their immediate background was Christian, the result of missionary campaigns in the 19th century, they too believe they are the remnant of Jewish refugees who reached India many centuries earlier. Some of their customs, burying rather than cremating their dead, and eating beef (anathema to many Hindus), they regard as evidence of their distinct pedigree.

In a book on the Bene Ephraim, The Jews of Andhra Pradesh, the British university anthropologists Yulia Egorova and Shahid Perwez observe that the group comes from the lowest caste of "untouchables", who have suffered discrimination in Indian society. Sadok Yacobi, founder of the community with his brother Shmuel, according to the authors, wants to "(re)claim the Jewish tradition as a means of social liberation".

Shmuel's elder son Yehoshua, who visited Israel with his father, went on to yeshivah there, formally converted to Judaism and met his wife, from Kiev, on an MA course in Bible at the Hebrew University.

While Western Jews fret about assimilation and, beyond the Charedi community, a decline in population, elsewhere people are bidding to become fully fledged members of the house of Israel. Whereas Judaism has eschewed evangelism for the better part of two millennia, the "lost tribes" are staking their claim to be part of it of their own volition.They may be captivated by the romance of the return of the Jews to their ancestral homeland, a seeming fulfilment of biblical prophecy, as well as a view of the modern state of Israel as a beacon of scientific and economic progress.

But as more and more of them are welcomed into the fold, the map of world Jewry may be about to change.