YOU NEVER KNOW where it’s going to come from. That sudden explosion of talent, that spark that lights up an art form. In the case of British ballet, it came from Ceausescu’s Romania, one of the grimmest spots in the Eastern bloc. On the day in December 1989 when Nicolae and Elena Ceausescu were put up against the wall and shot, an eight-year-old girl was outside Bucharest in a gymnastics training camp. Less than 12 years later, that girl was the toast of Covent Garden, freshly promoted to a principal with the Royal Ballet at the tender age of 19.

Alina Cojocaru’s life has been remarkable. From Bucharest to Kiev, where she trained at the state ballet school; then seesawing between Kiev and London until finally, in 1999, arriving in the corps de ballet at Covent Garden. Her rise has been meteoric, filling in at the last minute for an injured dancer in Ashton’s Symphonic Variations and catching everyone’s eye. Then, in rapid succession, Juliet and Giselle. Last season, she ate her way through the repertoire: fun-loving Kitri in Don Quixote; romantic Tatiana in Onegin; and this season her astonishing debut as the dangerous Mary Vetsera in Kenneth MacMillan’s Mayerling. On Monday night she dances Odette-Odile in Swan Lake, the acid test for a classical ballerina. The critics drool; audiences are in ecstasy; the superlatives fly: is there anything this young woman can’t do?

Cojocaru, now 21, is remarkably blasé about her extraordinary career, and the accelerated pace of a life that has packed in enough for someone twice her age. “I didn’t realise what was happening as it happened,” she says simply.

“I try to deal with the pressure by not letting it overcome me. Why should it when I’m trying to do my best? Dancing is the most important thing in my life. It makes me feel alive.”

If you passed her on the street, you’d mistake her for a schoolgirl — diminutive and unworldly — but when you meet her you are taken aback by her composure. For someone forced to carry the Next Big Thing on her shoulders, she is surprisingly unruffled.

Her size is misleading. Cojocaru may be tiny — so tiny her parents worried she wasn’t growing fast enough — but get her on stage and she’s one of the biggest dancers I’ve ever seen.

Her determination is in inverse proportion to her physique. How else could she drive herself to study ballet in a foreign country in a foreign language without the comfort of her family? She was just nine when she left her Bucharest home to train in Kiev. Lessons were in Russian; she spoke not a word (a translator hovered near by to explain the moves), yet within three years she was studying history and mathematics in her acquired tongue.

“I had never seen a ballet when I took my first class,” she says. “The whole thing made no sense to me.” Her English is fluent; she learnt it watching US programmes on satellite TV.

At the age of 16 she won a scholarship to the Royal Ballet School in London, but she stayed less than six months, lured back to Kiev by the offer of a job as principal dancer with the company there. In between rehearsals and performances she did her homework, determined to finish her schooling and gain a diploma.

That year in Kiev she had starring roles in Coppélia, Sleeping Beauty and Don Quixote, parts other dancers wait years for. But she had her eye set on the Royal Ballet, and when Anthony Dowell, then director, offered her a place in the corps de ballet — the lowest rank for a dancer — she decided to risk demotion.

In 2000, just months after arriving at Covent Garden, she was plucked from the corps to take over in Symphonic Variations at five days’ notice. It was her first Ashton ballet; she mentally rehearsed it on the Tube as she made her way home to her Streatham flat after a day in the studio. “I knew this was my chance. I felt quite honoured, it seemed very special.”

In 2001 she filled in for another dancer in Romeo and Juliet — she had a week to learn Juliet. “It was one of the great evenings of my life. The whole company was in the wings.” Giselle came next, and so impressed was Dowell that he promoted her to principal, right there backstage, after a performance in April. Cojocaru was stunned, but ready. “It was tough.

At one point last season I was learning five roles in one day with just half an hour between them, But I’m in the right place. It’s what I’m meant to do.”

The ructions of last season, caused by Ross Stretton’s short-lived directorship, made little impression on Cojocaru. “We were so involved with things at the end of last season that I didn’t realise this company wasn’t happy until I read about it in the newspapers. It’s hard to keep everyone happy in a big company like this. I had enough work, I didn’t rush to make any conclusions. After all, I was having a good time. It was my first year as a principal.”

She loves the MacMillan repertoire, with its flawed, passionate heroines, and looks forward to her debut as Manon later this season. “You completely abandon yourself, you put yourself at risk in these roles. If you don’t go for it what’s the point? To be able to be someone like Mary Vetsera . . . I feel I’ve lived her life.”

Mary Vetsera could prove the turning point for Cojocaru, the role nobody thought she could do. Too sweet, too naive: how could she possibly tap into the sadomasochistic cruelty of Rudolf’s young mistress? “It has been one of the biggest challenges; she is so not me. I could see in everyone’s eyes that they felt that too, that they were saying, ‘She can’t make it her own’. People can’t see me as Mary. You need experience in life to be a good dancer but not that kind of experience.”

The frightening thing about Cojocaru is how fast she’s matured as a dancer, and the worrying thing is that at this level of intensity she will burn herself out before she’s thirty. We’ve seen it happen before. But Cojocaru is as adamantly secure on this point as she is on every other. “I have to dance everything and I can’t save myself for the future,” she says. “If I cut my career short as a result then so be it.”

• Swan Lake opens at Covent Garden (020-7304 4000) on Monday