The discovery of a tunnel dug by yeshivah students under the headquarters of Lubavitch HQ in New York is the latest bizarre chapter in one of the more extraordinary controversies in modern Judaism: over whether the seventh and last Luvabitcher Rebbe, Rabbi Menachem Mendel Schneerson, who died in 1994, was the Messiah in waiting.

The charismatic revivalist launched the outreach movement which has inspired thousands of emissaries to set up Chabad Houses around the world, including in the unlikeliest of places, and touched the lives of millions of Jews.

But while the official Lubavitch movement has distanced itself from claims that he incarnated the spirit of the Messiah who will return to be revealed, his openly messianic supporters - known as Meshichistim - believe that their views are much more widely shared; for them, the fact that no successor to Rabbi Schneerson was ever appointed is proof.

It’s believed that the young men who dug the tunnels underneath the HQ in 770 Eastern Parkway were Meshichistim – set on expanding 770 according to their interpretation of the Rebbe’s wishes.

Belief in a Messiah is one of the cardinal tenets of traditional Judaism, forming one of Maimonides’s Thirteen Principles of Faith. According to the prophets, he will lead the exiles back to Israel and oversee the rebuilding of the Temple.

But the exact nature of the Messiah and the circumstances of his arrival have been a source of debate since talmudic days. In one tradition, every generation has a potential Messiah who could be unveiled.

In the educational campaigns he initiated, Rabbi Schneerson certainly put emphasis on messianic belief, encapsulated in the popular chant “We Want Moshiach Now”.

The Meshichtim say that recognition of the Rebbe as Moshiach reflects his wishes but mainstream Lubavitchers reject any such speculation.

Although the messianic faction is dismissed as a fringe, critics of Lubavitch argue that belief in the Rebbe’s messiahship enjoys much broader currency.

A small community centre was started in London by one open supporter of the Meshichistim over 20 years ago, in Stamford Hill. But in his 2002 book, The Messiah Problem, Rabbi Chaim Rapoport, the former minister of Ilford Synagogue, wrote: “The Lubavitch schools and synagogues in England do not allow any messianic rituals, slogans or songs to be practised in their territories. This marginal group has been all but ‘excommunicated’ by mainstream Lubavitch leadership.”

But periodically, trouble breaks out. One argument in a New York yeshivah led to fisticuffs and police arrests. In one of the wilder displays of pro-messianic fervour, an Australian group held a se’udah, a celebratory meal, on one of the fast days held to mourn the loss of the Temple.

The Rabbinical Council of America, the country’s mainstream Orthodox umbrella body, has banned membership to anyone who proclaims the Rebbe as Messiah.

An Israeli rabbinic court has even refused to recognise the Jewish status of a pro-Meshichist convert.

Criticising the Israel decision, Rabbi Shmuley Boteach, one of the world’s most famous Lubavitchers, wrote that Chabad’s Messianics were mostly Jews “with a deep spiritual orientation who desperately wish to see the world cured of its ills. Their mistake is to allow that yearning to spill over into desperation and to ignore the 3,000-year-old Jewish insistence that the Messiah be a living man.

“Indeed, most Lubavitchers I know who insist the rebbe is the Messiah do so more out of a visceral, emotional attachment to the Rebbe’s memory than out of any deep-seated halachic conviction.”

One such Messianic was the late Moshe Yess from Montreal, a musician turned Lubavitcher who in the 60s had shared a stage with Jefferson Airplane. In a letter to the Jewish Chronicle in 2008 challenging anti-Meshichist correspondents, he said that the Rebbe “made it clear he is a prophet” and had identified himself as Messiah “by various means”.

His song, “The Rebbe of Lubavitch is Messiah” can be heard on Youtube.