Questions were asked in the French parliament in February 1989 when Sylvie Guillem left the Paris Opera for The Royal Ballet. Le Monde called Guillem's defection 'a national catastrophe'. At Covent Garden, Guillem would be Principal Guest Artist. This was a new title; Nureyev and Natalia Makarova had danced regularly with the Company, but always as guest artists. Guillem had a firm contract that gave her control over her repertory.

Sylvie Guillem

Now the ballerina who brought glamour, wit and brilliance to the Royal Ballet, is leaving to devote the rest of her career to modern dance. At 42, she has danced virtually every major role in the ballet repertory and her standards are too solicitously high for her to relish in repetitive performances, which would in time expose the inevitable, slow diminishing of her classical technique.

Ever since she was promoted to etoile of the Paris Opera Ballet at the dazzlingly young age of 19, Guillem has been the superballerina of our time, celebrated not only for the fortuitous perfection of her body, but for her unprecedented glamour she radiates on stage. Her fans are so obsessive they trade details of her performances over the internet.

Her body looks as if it could go on forever. Still preternaturally slender and supple, it has thrown very few injuries at her, and Guillem says she has got much better at managing the chronic pain that is a daily fact of her dancing life.

Guillem is so dedicated to giving herself new challenges that sometimes she gives the impression that taking risks has motivated her more than a simple love of dance. Yet she says the two are inseparable. "Working with new people is what life is about for me - it is like confronting a new country, a new vision". She cites her recent encounter with Lin Hwai-min, the Taiwanese choreographer with whom she and the Kathak-schooled prowess of her partner, Akram Khan have been collaborating on part of the material for Sacred Monsters. "This little man of 60 was explaining some movement. It looked at first very simple, almost banal - but the feeling he had in these movements, the tradition he was coming from, were so powerful it meant I was having to learn to use my eyes completely differently. Moments like that are fantastic - you are given a new perspective, not just on your job, but on life".

Behind the professional brilliance, Guillem is a fiercely private person, not the kind who is on television and in magazines every five minutes selling cars and washing machines. Although a few years ago, when she agreed to do a photospread in Vogue, she typically turned the assignment into an act of artistic provocation, posing stark, non-airbrushed, naked, and more shockingly still, refusing to wear a single lick of make-up.

In 2003, Guillem, the professional rebel, who for years danced under the nickname 'Mademoiselle Non', was appointed by Her Majesty The Queen to Honorary Commander of Most Excellent Order of the British Empire (CBE).

Darcey Bussell

Now the country's favorite ballerina is quitting while at the height of her profession. Darcey Bussell, the darling of Covent Garden will be hanging up her pointe shoes for the last time on Friday night. BBC2 will broadcast the bittersweet event live to the nation along with a tribute documentary. Steadfast as ever, Bussell will exit from Covent Garden not in one of her established roles, but after a performance of Kenneth MacMillan's Song of the Earth, based on Mahler's Der Abschied (the farewell). Mahler's piece is somber as it is about the departure from life.

However, ballet fans are not convinced and predict the inevitable Covent Garden comeback. Immersed in the world of ballet for the last 28 years, her followers believe that her passion and dedication will be her calling. No dancer of our times has connected more naturally both with the dance audience and with the wider public.

Strictly speaking, Friday's farewell will not be Bussell's final curtain call. In the weeks before Christmas, she will emerge in a new - and to ballet purists, artistically questionable - guise to tour the country in a series of shows with crossover singer Katherine Jenkins entitled Viva la Diva. In these unashamedly populist performances, there will be little or no ballet. Instead, Bussell will tap-dance in public for the first time.

Seeing Bussell in rehearsal is confirmation that the qualities that have made her special are still in perfect working order. Her anatomical features define perfection. Uncompromised training has produced a distinct fluidity of movement. Bussell's gestural movements are wrenching and drive you to a sense of elicitation.

Bussell was a wonder at 19, a tall, questing, adolescent beauty in a blindfold, plucked almost fresh from school to a star in Kenneth MacMillan's ambiguous 1989 fairytale The Prince of the Pagodas. The milkmaid grace with which Bussell negotiated his sexually charged choreography with four men she couldn't see, made it radiantly clear that this girl should be the next great Covent Garden ballerina.

But three years later, MacMillan was dead, and the Royal Ballet's youngest principal, too tall for most men, lost momentum and confidence against the sensational, tall Sylvie Guillem and the tiny, quicksilver Viviana Durante.

Then Bussell learned to exploit the competition. Some of Guillem's tall partners were delighted to double-up with the amiable Darcey. On the Royal Ballet's American tours the public exclaimed at this big, angelic girl with a thrillingly high jump. Being hyped as the new Great British Ballerina did her and the Royal Opera House no harm, nor did 'Mademoiselle Non' Guillem's indifference to the media spin that whipped up a balletic version of the age-old feud between English goodies and French baddies.

Bussell's decision to quit ballet now while on such seraphic form, may yet go down as one of the most inspired decisions her profession will ever see.

Bussell was awarded a CBE in 2005.

Darcey Bussell's Farewell: Live is on BBC2 Friday at 9pm.