There is a fine art to upstaging headline acts, and Bi Kidude has had more than 80 years to perfect it. A singer of music that filled an ancient need for doctor, big sister and sex therapist, she is the uncrowned queen of Zanzibar, the Indian Ocean idyll that contributes the “zan” to Tanzania, and nobody there needs to be reminded of her significance as an emblem of the old ways.

Watching her entrance at last year’s Sauti Za Busara festival in the old Arab fort in Stone Town, the island’s labyrinthine capital, was an education for anybody tired of a world in which pop celebrities spill out of limousines. Each evening she arrived in a fine one-piece robe, a matching scarf tight around her shaven head, a king-size cigarette permanently in her left hand and gold bangles around her arms. Imagine the Queen impersonating Bet Lynch.

She would then march to the VIP seating area so that the festival organisers could line up with hotel owners and musicians to offer praise and brave her iron handshake. Schoolchildren were led en masse to sit at her feet. Even on nights when she was not performing, for 30 minutes the show belonged only to her. Anybody planning to play this year’s Sauti Za Busara (February 9-14) be warned: you are just a warm-up act.

She remains unknown outside her homeland to all but the most attentive fans of African music. In 2005 she won a lifetime achievement award at an international worldmusic gathering and took to the stage with words “I drink, I smoke, I don’t need a microphone”, before delivering a stunning a cappella set.

Somehow, however, 2007 might be the year of Bi Kidude, and not before time. Her musical career began in the 1920s and she claims to be approximately 113, although other islanders scoff at the notion. “She’s not a day over 94,” they insist.

“I was born in the time of the rupee, not the shilling, in the time of Sultan Khalifa,” she states categorically, though with such a determined lack of precision that a new documentary about her has had to adopt a diplomatic title: As Old as My Tongue: The Myths and Life of Bi Kidude. A new compilation album, Zanzibara 4, puts forward a forthright argument for your attention in its subtitle: The Diva of Zanzibari Music.

Whenever she was born, Fatuma Bint Baraka was the daughter of a coconut seller on the island of Unguja and was seduced by the nightlife in Stone Town, a thriving port on the spice routes. After a failed marriage in the 1930s, she moved to the mainland, singing with the Egyptian Music Club, combining Arab instruments (the oud and violins) with African flutes and drums in a new form, taarab, which represented East Africa’s role as a crossroads.

In the 1950s, however, she returned home and retired from professional singing to concentrate on a career as a traditional healer. Part of her new life involved educating girls and young women, using song rather than lectures. The public face of her music was msondo, performed at wedding ceremonies, in which the singer would give the bride-to-be a guide to keeping her man happy. She also developed a reputation for playing a raunchier form of music, unyago, at private initiation ceremonies where women learnt the facts of life. “The wife has been busy preparing doughnuts,” she sings on Jua Toka. “A mouth without teeth, how can it chew the pastry?” The 1950s and 1960s saw Zanzibar turn in on itself, with political incompetence bringing the economy to its knees until the 1980s. Once the island began to reclaim its place in the world, politicians would talk about preserving the prewar culture, and Kidude’s name would come up as the last of her generation.

There then began a race to preserve her music, songs that had never been recorded and were often so ribald that they had scarcely been discussed in polite Islamic society. Zanzibara 4 rounds up the best, from sessions with the Zanzibar Taarab All Stars in 1989 to recordings made last year. It would be disingenuous to say her voice is easy listening, but 80 years of smoking and drinking will do that to anyone. “How can I stop singing?” she asks when approached about retirement. “When I sing I feel like a 14-year-old girl again.” She was never going to give the youthful Charlotte Church a run for her money.

But it is on the unyago songs that she comes alive, and her show is something to behold. Wearing a peasant shirt and a sarong hitched up so she can straddle a 4ft drum, she picks up a beat that she and her fellow drummers hold unrelentingly for as long as an hour while she barks out advice. “To be old does not just mean that the teeth fall out, but it is good if his machine is still working.”

In front of her dance a group of women who make explicit the gist of her words with a booty-shaking demonstration of what goes on behind closed doors in what appears, on the surface, to be one of the world’s more reserved societies. At Sauti Za Busara, what was once a lesson in love for young Zanzibari women now grips everybody.

“I still make people happy,” she smiles, and takes a long drag on her cigarette. “And I’m still alive.”

Zanzibara 4 is out on Jan 20 on Buda/Discovery.

Nonagenarian Bi Kidude could be the world's oldest singer. If you know of anyone who could rival this claim, then let us know by using the form below.