Two years ago, she sang plaintively that she was "Not a girl, not yet a woman."

For the past couple of months, former pop princess Britney Spears, 21, has been out to prove that girlhood is but a distant memory, posing topless and bottomless for magazine covers and smooching publicly with Madonna to promote her recording comeback.

In the Zone, Britney's first album for two years, hits record stores this week following her decision last August to take a break from the business that had turned a wholesome teen from the American South into a worldwide pop phenomenon with album sales of 60 million.

Two years is a lifetime in the fickle pop music industry - as Madonna, the mother of reinvention, knows all too well.

So once the TV specials, the eye-popping photos and the media furore over her "not that innocent" new image blows over, Britney's success as a recording artist will depend on the music.

"It really comes down to how good the songs are. Any kind of image is not going to sustain you for that long unless there is really some substance there," said Jeff Pollack, a leading American radio and music consultant.

"What really matters in a music career is getting played on the radio, on MTV, getting legitimate acceptance as an artist.

"Britney faces a transition now. In terms of phase one of her reinvention, it's working," Pollack said.

He expects the new CD to have a strong first week and to make it easily into the Top 10.

But he adds: "If the record in not in the groove then programme directors at music channels are not going to keep playing it".

The first single from the album, Me Against the Music, boosted by a steamy video featuring Britney's new pal Madonna, has been playing fairly well on radio.

Britney's stripped-down new look, together with the revelation that she is no longer the virgin she once professed to remain, has moved her away from her former teen fan base which is embracing Disney TV star Hilary Duff, 16, and Canadian punk singer Avril Lavigne, 19.

Her overtures to the young adult audience put Britney up against the likes of Beyonce Knowles, who is enjoying a runaway success both as a solo artist and actress, and megadiva Jennifer Lopez, whose love life and revealing outfits have ousted Britney as the world's most talked about celebrity.

Music has also moved on, with rap and R&B singles dominating mainstream US radio playlists where five years ago bubblegum pop from pigtailed girls (and boys) next door reigned supreme.

Britney's comeback promotion has been breathtaking. Her sensational open-mouthed public kiss with Madonna on the MTV Music Video awards show in September made world headlines.

At least three American TV specials will mark her comeback. She posed topless on the cover of Rolling Stone and wore nothing but a white sweater and high heels for Esquire.

Esquire reporter Chuck Klosterman said Britney, while not commenting on whether she saw herself as a sexual icon, "acted as if this had never crossed her mind. Either Britney Spears is the least self-aware person I've ever met, or she's way, way savvier than any of us realise."

Whether or not In the Zone is an artistic success, personal branding consultant Peter Montoya said Britney's cultural status makes her a "marketing powerhouse".

"She has been so well marketed that the question of her talent is really a non-issue,"he said.