Sugababes are a genuine pop phenomenon. The most successful female act of the decade, they've notched up five number ones and 18 hit singles since their arrival in 2000. More impressive is the fact that they've combined commercial success with credibility, in a way that harks back to the great girl groups of the Sixties. They're poppy, for sure, but striking, edgy, moody. And until she surprised everyone by announcing her departure, one member, Mutya Buena, summed up the urban attitude of the group more than her two bandmates.

Mutya was the band's driving force and most recognisable member - partly for her distinctive vocals, partly for her fantastic "woteva" pout. She was a Sugababe from the beginning, joining up in 1998 at the age of 13, but walked out after the birth of her daughter Tahlia in 2005, which compounded her reputation for being "difficult".

She was soon replaced - her old bandmates recently made number one with Walk This Way, a Comic Relief collaboration with Girls Aloud - but Mutya is out to prove that she was the talented one all along as she launches her solo career. Her single, Real Girl, is due next month, with an album to follow in June. Is she more willing to drop the pout this time around?

"I was really young when we first came out," she says now. "We didn't do any media training. So when people were trying to talk to me, I was sullen. I should've gone, 'I need to make myself better at this'. But instead, I got angry and I'd make myself worse. I'd stop talking to everyone. I'm very hard-headed. I'm a Taurus. I blame it on that."

Despite her reputation, the Mutya I meet is a self-deprecating 21-year-old, a wit who answers every question honestly (her favourite conversational link is "to tell you the truth"), and is extremely likeable. She is relishing her new freedom, away from the strict regimes of Sugababes.

"The relief of all this is not having to answer to anyone. Not arguing over who wears what top, as girls do. And making your own decisions about music. Having control of your life. Because our lives were still being controlled. We'd still have to do things we didn't want to do or sing things we didn't want to sing.

"Obviously, there'll be some 'She's not as good as before', and all that. But I'm prepared for that. Nothing's going to stop me doing what I want to do. I can't wait."

Casual fans will be surprised by the raw toughness of Mutya's solo material. The Real Girl single, based on an orchestral sample from Lenny Kravitz's It Ain't Over 'Til It's Over is instantly memorable, a surefire hit.

The album of the same name features adult R&B ballads such as Suffer For Love, Wonderful and Paperbag, which allow her to show off her strong, sultry voice, and sit comfortably alongside some joyful, pure pop collaborations.

She recorded Out of Control at Groove Armada's house, and teamed up with Amy Winehouse on B-Boy Baby: "If that gets released as a single and we make a video, the amount of stupidness me and her would come up with - oh my God! That's if we even both turn up."

On This Is Not Real Love she teams up with George Michael - a former pupil of her old school in north London, Kingsbury High. The duet, already released as a single, hit the Top 20 in November last year and puts her in a select club with his former female singing partners Aretha Franklin, Mary J Blige and Whitney Houston. Mutya performed the number with Michael on tour last year.

One of eight children, brought up in Kingsbury by her Irish mother and Filipino father, Mutya's love of singing suddenly became a serious ambition after a chance meeting between her dad, who owned an Oriental supermarket, and one of his customers, a pop manager who had already discovered future Sugababe Siobhan Donaghy.

Mutya auditioned and found herself with professional management at the age of 10. Mutya was 11 when she, Donaghy and Keisha Buchanan formed Sugababes; 12 when they were signed to London Records; 14 when the band's debut single, Overload, was released; and 16 when, after disappointing sales of Sugababes's debut album, One Touch, London kept Donaghy and disastrously opted to drop Sugababes from the label.

Undeterred, the band replaced Donaghy with Heidi Range (an ex-Atomic Kitten) and found a new label, Island, before Freak Like Me, a superb melding of Adina Howard's R& B hit and Gary Numan's Tubeway Army classic, Are 'Friends' Electric?, made Mutya one-third of Britain's biggest girl group. She was still only 17.

The pop hot-housing meant that Mutya's education essentially ended at the age of 13. "I kind of wish I'd stayed at school," she admits. "It was very hard. We had to leave school, get private tutorials. We were being pushed all over the place to sing, for schooling, all sorts. Then you'd go, 'I wanna go to the Valentine's dance at school', and you couldn't go because you'd left. The regret is that I could've been and done all the things that the rest of my friends were doing. I went two years without seeing any of my friends."

Sugababes had become her life. Were there any hard feelings with her former bandmates when she abandoned them? "Obviously, me and Keisha go back to childhood and Heidi was with us for four years - and for a year I didn't speak to any of the girls and I haven't spoken to Heidi at all. She obviously found it hard. There was one article where they said I'd used my daughter as an excuse to leave the band. She wasn't an excuse. She was a reason."

Tahlia is now two, and, whatever happens with the new album, Mutya plans to stay in Kingsbury, with Tahlia's father, her long-term boyfriend Jay, keeping mum and schoolfriends nearby.

"They're the people who keep me grounded. Without grounded people around you, you just end up being a right prat."

There's every chance that her solo career will be as successful as the Sugababes, but it looks as if Mutya won't be making the same mistakes second time around.