"Wow, wow, wow... Talk about a star is born. You're the real deal, girl." It was with these words that Oprah Winfrey bestowed her blessing on Leona Lewis.

Following a live performance on her chat show last week by the talent show winner from Hackney, east London, the all-powerful Winfrey advised viewers they could download Lewis's single "Bleeding Love" from iTunes, or buy it from Target record stores.

Americans did so in their droves and yesterday Lewis's single shot to No 1 in the Billboard Hot 100, knocking Usher from pole position " the first British woman to top the US singles charts since Kim Wilde in 1987 with her cover of The Supremes' "You Keep Me Hangin' On".

Lewis, 22, a former receptionist and pizza waitress, who has earned comparisons with Whitney Houston, Mariah Carey and Celine Dion, is also the first winner of The X Factor to make it big on the other side of the Atlantic. 

When she won the ITV talent contest in 2006, the judge Simon Cowell said, in an uncharacteristically gushing critique: "For every little girl who dreams about being a pop star while they're working in an office, you're a role model."

But British music critics have not always been so kind, dismissing Lewis's music as too mainstream. Despite being nominated in four categories at the Brit Awards last month, Lewis walked away empty-handed.

The Billboard endorsement might force some of those who have sneered at Lewis's success in the UK to revise their opinion.

Only two other UK female artists in history have topped the US chart with their debut release - Petula Clark in 1965 with "Downtown" and Sheena Easton in 1981 with "Morning Train (Nine to Five)".

Lewis has even trumped Amy Winehouse, who, despite winning five Grammy Awards, has not yet achieved a No 1 in the US. It is Cowell's involvement above all which has helped to catapult Lewis to fame in the US. 

As a judge on American Idol, the US version of The X Factor, Cowell is one of the most high-profile Brits in America. Through his record label, Syco, in tandem with Sony BMG, he has the first option on all winning acts to emerge from The X Factor. 

While previous winners have sunk without trace, Cowell spotted something different in Lewis, describing her as the most talented singer he has heard in more than a decade, and did the groundwork for her to become a global star.

In 2006, Lewis enjoyed a Christmas No 1 hit with "A Moment Like This", which broke the world record by selling 50,000 downloads in its first half-hour on sale and went on to sell 600,000 copies in its first week.

Realising that her success was dependent on the right material, Cowell contacted Clive Davis, the founder of J Records  also part of Sony BMG  and told him: "You might have the next Whitney Houston on your hands."

Davis, the man who discovered Houston and Alicia Keys, agreed with Cowell's judgement: "I was immediately knocked out by her range, her versatility, and the pure beauty of her voice. She is an artist who will be a true star for many years to come."

Lewis signed a recording contract with Syco in the UK and with J Records and Syco in the US, earning a reputed ?5m advance, and started work on her debut album, Spirit, working with musicians who have previously played with Madonna, Gwen Stefani and Beyonc?.

For nine months, Lewis went quiet as far as her fans were concerned. Then, in September 2007, she re-emerged at a VIP music industry event in London, where she previewed some of her new material, including the single "Bleeding Love". She went on to premiere the single on her old stamping ground, The X Factor, on 20 October last year. Two days later, the track became the biggest-selling week-one single of 2007, shifting 218,805 copies in seven days.

In November, Lewis's Spirit was released in Britain and became the fastest-selling debut album of all time in the UK, with 375,872 copies sold in its first week, breaking the previous record set by the Arctic Monkeys, who sold 363,735 copies of Whatever People Say I Am, That's What I'm Not in January 2006. This week's Billboard triumph suggests that when Spirit is released in the US on 8 April, Lewis will enjoy a similar level of success.

As Lewis told Winfrey, she was always destined to sing. "I remember singing into my hairbrush and every chance I could I'd be doing a show or performing in the front room for my family."

Tall and slender, with a tousled mane of brown hair and sultry green eyes, Lewis's stunning looks come from her half-Italian, half-Irish ballet teacher mother and her Guyanese father.

As a young girl in Hackney, Lewis already had one eye on stardom. She attended the Sylvia Young Theatre School and later the Brit School in Croydon, a crucible for the current generation of young British music talent  the state school counts Kate Nash, Adele Adkins and Winehouse among its alumni. At the age of 15, she sent a demo tape of herself singing Minnie Riperton's "Loving You" to Sony in America, but at that time the record giant failed to sign her up. After leaving school, Lewis worked as a waitress at the Stamford Hill branch of Pizza Hut and as a receptionist, to earn money to pay for studio time.

A teetotal, animal-loving vegetarian, Lewis still lives in a rented flat in Hackney with her boyfriend Lou Al-Chamaa an elctrician. But for the rest of 2008, Lewis is unlikely to be spending much time at home. She kicked off her assault on the US last month with a performance at Clive Davis's pre-Grammys party, which won her critical acclaim and a profile in US Weekly. 

Over the next few weeks, she will continue her promotional tour of the US, which has already included an appearance on NBC's Tonight Show with Jay Leno, with interviews scheduled for The Ellen DeGeneres Show, ABC's flagship breakfast programme Good Morning America and Jimmy Kimmel's late night talk show. Then it is on to Australia, one of the 10 other countries where "Bleeding Love" has topped the charts, followed by Asia.

All the overseas attention will do Lewis no harm back in the UK. An HMV spokesman, Gennaro Castaldo, said: "There was always a slight cynicism over artists that emerged through the reality TV process, but they've developed a truly global star here, not least because Simon Cowell has such a massive profile in the States that he really can open doors. Few people would dispute that she does look amazing; she has an incredible voice; and she's working with the right recording industry people over there, so if you give her the right material, that's a winning formula in the US.

"It's a virtuous circle which will then spill back to the UK. If there's anyone with an outstanding cynical attitude towards her, this will convince them she is more than an X Factor winner."

Despite finding her music "very safe" and "overwhelmingly mainstream", the Billboard contributor Paul Sexton agrees that Lewis has undoubtedly hit the big time. "There's no denying she's a big star now," he said. "Getting to No 1 in America is a very big endorsement, whatever you think of her music."

He was surprised that Lewis did not win at the Brits. "What that did is emphasise that the Brits are not stage-managed. Everyone expected her to walk away with at least a couple of awards. The performance she gave was powerful on the night."

Of Lewis's US success, he added: "Simon Cowell has become a huge celebrity in his own right on American Idol; that has got to be a huge factor in promoting a new act."

Cowell said he was "incredibly proud" of Leona. "What she has achieved in the last year is simply amazing." Speaking on Winfrey's programme, Cowell described his prot?g?e as "one of the nicest people I have ever met in this business".

In the first of a series of blogs on the US teen website Cosmogirl, Lewis lived up to this description with a breathless account of her US tour. "I'm so excited to be able to spend some time in the US and showcase my work to everyone here. It's been a dream of mine ever since I was a little girl and I just can't wait! Thanks to everyone for the support so far. It's just incredible and I appreciate it so, so much."

She may have been compared with some of pop's greatest divas, but, thus far at least, Lewis exudes the charm of a pleasant and modest young woman, albeit with a star quality and stunning voice that has propelled her to international fame.