Click here to watch the video for Sing: including footage of Annie Lennox's trip to Africa

If you didn’t listen closely to the words, you would hardly guess at the grim subject matter that inspired Annie Lennox’s new single, Sing. A bouncy pop-soul anthem reminiscent of the Eurythmics classic Sisters Are Doin’ It for Themselves, it sees Lennox leading 23 of the world’s most famous female pop stars – a cast headed by Madonna, Céline Dion, Shakira and Sugababes – through a tune that, on first hearing, sounds like a clamorous invitation to a rollicking hen night.

In fact, Sing is probably the most serious song Lennox has ever recorded. After years spent supporting a range of charitable causes, from Amnesty International to Children in Need, she has become passionate about the Aids pandemic that is sweeping southern Africa. She is appalled that the disease now hits women - especially pregnant mothers and their unborn children - harder than it does men, and has a shocking statistic to report: latest estimates suggest that as many as one in three mothers-to-be in South Africa carries the Aids virus. That most are unaware of their condition was what prompted her to write a song about it: “For years, I’ve been wondering how I can maximise what I do to tell people about HIV in Africa,” Lennox says. “With Sing, I’m encouraging women at risk to find out about their status because there’s such a stigma about talking about Aids in the places where it’s most rife.”

Though the attention of most western listeners will focus on the largest stellar gathering captured on record since the Band Aid/USA for Africa singles Do They Know It’s Christmas? and We Are the World, for Lennox the key to Sing lies with the female township choir at the end. This is a group of Aids activists, the Generics, singing in their native Xhosa, harmoniously pleading to be given the antiretroviral drug AZT. “I thought it would be great if I could take a tune from our musical culture and place it next to theirs,” Lennox says. “I don’t believe I’ve written anything as great as Paul Simon did on Graceland. But I’m trying my best.”

It seems appropriate that, for today’s interview, Lennox is dressed in black, a colour that suits her theme and lends gravitas to her chiselled, alabaster-white features. Her welcoming smiles, rather like the upbeat demeanour of Sing, belie the emphatic earnestness of everything she says. Though she is an attentive hostess, pouring the tea and offering shortbread biscuits, she doesn’t do small talk. Any questions that veer away from the topic that is uppermost in her mind are swiftly redirected.

While admitting that she leads “a very comfortable life, as you can imagine”, feeling uncomfortable seems to be her default mood these days. Now 53, Lennox lives with her two daughters, 17-year-old Lola and Tali, 15, in west London. Divorced for eight years, she is currently unattached, and her most important relationship, outside the family, appears to be the knotty one with her social conscience. After a hugely successful career spanning nearly three decades, during which she has sold more than 80m albums, a life of ease is not an option. Lennox says she views the world as “a pretty dark f***ing place”. Even, apparently, when she is in the bathroom. “When I’m having a shower in the morning, or brushing my teeth. I think, ‘This is easy, I just turn on the tap.’ And every time I do, I have a picture in my mind of these children and women in Africa walking for miles, usually barefoot, with canisters on their heads, carrying bundles of damp wood to heat the water.”

As Lennox sees it, they are the lucky ones. Since 2001, when she met Aids victims at a hospital in the Eastern Cape province and saw the chronic shortage of medication to prevent the transmission of HIV from pregnant women to their babies, Lennox has been searching for ways to bring the problem to wider attention. She initially sought advice from her old friend Bob Geldof, who wasn’t much help. “Bob’s message was, ‘Just f***in’ do something and see what happens. I can’t tell you what to do.’”

Lennox started by returning to Africa to make short films. “Because people have such a short attention span, I thought I’d do these documentary soundbites that would make them go, ‘Whoa, bloody hell.” She would film heartrending emblems of the Aids cull, such as the stacks of tiny coffins on sale in supermarkets in slum districts. For her, the most poignant clip was of a girl of seven, Avellila, whose mother had died, leaving her in the care of her ageing grandmother. When Lennox first met her, she weighed less than a one-year-old. “She was skeletal, with pneumonia from full-blown Aids.” Although the doctors were expecting her to die at any time, thanks to Lennox’s intervention, “the day after we arrived, she started to get better. It was weird. Five months later, she looked like a normal healthy seven-year-old, even though she can’t be fully cured. She’ll be on antiretrovirals for the rest of her life”.

The idea to incorporate such experiences into her music occurred to Lennox through the contact she made in the Eastern Cape with the pressure group, Treatment Action Campaign (TAC). “They’re a grass-roots collective,” she says. “They’re not wealthy people, they’re just trying to promote awareness among women about the simple practical steps they should take.” In 2006, a TAC representative visited Lennox in London and gave her a CD of the organisation’s house choir, the Generics - “They find singing is the best way to get their message across in South Africa.” Shortly afterwards, she sent an e-mail to 30 fellow musicians and received 23 positive replies. “Which wasn’t bad.”

Lennox discourages any diversionary celebrity gossip about who said what - and why bossy-boots Madonna exceeded her brief by singing the second verse rather than just the chorus, like everybody else. Lennox is also cagey about naming the no-shows, though she insists that Amy Winehouse, whose “blinding talent” she says she greatly admires, wasn’t one of them: “I don’t know why I didn’t ask Amy. No reason. She’s one I would probably go back to next time.” By which she means that she found it so straightforward to record Sing - by e-mailing a music file and asking the participants to record over it in their own time - that she hopes to repeat the experiment.

The only obstacle was the initial reluctance of Lennox’s record company, Sony BMG, to support her. Her original plan was to release Sing as a download to coincide with her performance of the song at the World Aids Day concert in Johannesburg last December. “When I got there, I discovered the record company hadn’t done anything at all to promote it, which was ...” she pauses, choosing her words with diplomatic care, “...disheartening for me.”

Back in London, she read the boss of Sony/BMG the riot act. He promised to make amends. “It’s all turned around now. They’ve got right behind it.” It was actually the company’s idea, she explains, to make the rereleased “hard copy” of the single - on CD this time - available in the UK exclusively through Body Shop outlets. “Which is a beautiful twist of fate, actually, because the last time I saw Anita [Roddick, founder of The Body Shop, who died last September], she was at my house and I played her Sing. She was laughing and saying, ‘I think this is fantastic!’”

Lennox is so full of talk of Sing, and the many underpublicised aspects of the Aids crisis in South Africa - “Not many people know the virus can be transmitted by breast-feeding” - that it’s easy to forget how tireless she has been, and continues to be, on behalf of other causes. Her appearances at events such as Live8 barely hint at the extent of her philanthropic interests. She has lent public support to some 20 charities, as well as helping to ease the homeless problem in the capital by buying up houses in north London and letting them, for a nominal rent, to former street sleepers.

Last month, Lennox was speaking out for Hear the World, a campaign to spread awareness about the deafness that afflicted her late father, a boilermaker in the Aberdeen shipyards. With her former partner in Eurythmics, Dave Stewart, she donated all of the profits from their 1999 reunion tour to Greenpeace. In 2000, she worked with Amnesty to secure the release of Ngawang Choephel, a Tibetan ethno-musicologist jailed by the Chinese government. She has been an ambassador for Oxfam and is the patron of an MA course in humanitarian and development practice at Oxford Brookes university. To judge from the title of her most recent album, Songs of Mass Destruction, the problems of the world seem to weigh ever more heavily on Lennox’s shoulders.

You do wonder why a woman who admits to having suffered from bouts of depression in the past chooses to dwell at such length on matters that might invite more visits from the black dog. Lennox responds briskly: “It would only be depressing if I thought there was nothing I could do to make a difference. In that case, I would be like some surreal tourist. You do this because you’re called by something within yourself. A deeper yearning, maybe. There are reasons why people get depressed, and one is that they think the world stinks. They’re not far wrong, actually. But feeling that way is a hole, and you’ve got to get out of it.” She seems to sense that the interview is swerving off message here, and reins back. “I’m not saying I’m doing this to get out of it.” She pauses, then adds an enigmatic full stop. “But my experience has informed me in such a way.”

Sing is available at The Body Shop and iTunes from tomorrow; it will go on general sale on March 17. Proceeds will be used to support TAC

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