At 48 she sells out dance houses around the world. On stage she is simply mesmerising. It is not only her long legs, which seem to stop only just before her mane of red hair. It is her unique stage presence that makes it impossible for an audience to take their eyes off her, her athleticism, her fearlessness to do the impossible with her body, perhaps a legacy of her childhood as an aspiring Olympic gymnast, her move from classical ballet to the most adventurous and experimental contemporary choreography, and in the background her reputation as one of the most strong-minded, wilful and demanding personalities the art form has seen.

It is a reputation that is somewhat tempered in my nigh-on two-hour chat with Sylvie Guillem, alone on a bench in a beautiful garden in Milan where she is staying, alone that is save for the presence of one of her two gigantic white Swiss/German shepherd dogs. She turns out to be open and engaging, not the diva of repute. And yes the French accent helps, as does the striking red hair enhanced by large, piercing green eyes. Her face, without a touch of make-up, is disarmingly radiant, but then she didn't wear any make-up in her one Vogue shoot. Oh, and she made a unilateral decision to be naked for that too, "They wanted a beautiful, gorgeous woman. What was the point? I wouldn't do it as good as the professionals. So I thought I will see who I am. I won't be disguised and transformed."

The former star of the Royal Ballet remains scornful of some of the biggest names in the dance world, but willing to answer any question, to justify her separateness, to discuss why she didn't have children, and more than eager to talk about her recently found espousal of a political cause Sea Shepherd to champion ecological awareness and biodiversity and warn against the threat to the oceans, the planet's fish and other species. Sea Shepherd is a younger relation of Greenpeace, stressing how (it predicts) the oceans will be empty of fish by 2048, and Guillem sits on its arts and media advisory board with Sean Connery, Brigitte Bardot and Martin Sheen.

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She is wearing a Sea Shepherd T-shirt, and audiences at Sadler's Wells in London where she appears shortly in 6000 Miles Away, a trio of dances that she commissioned, where she is an associate artist, will be made aware of the cause.

I ask her first about her time as Rudolf Nureyev's protégée at the Paris Opera ballet company. She must be one of the only dancers still performing who has danced with that legend. "Yes, but I never saw Rudolf dance well," she answers. "He was fairly old by then. But he had passion and curiosity and vision. He put me up on stage with him. He knew that dancers didn't have any time to lose. Some directors make you wait. We were very lucky to be there with Rudolf. He never hesitated to have around him young people, much more gifted in a way because of their youth. We did have big arguments. He was extremely shy, I was extremely shy. He had a terrible character, I had a terrible character. But I respected him a lot, He was one of only three people I have ever met in my long career that I could talk to about dance." I mention that I was at Nureyev's funeral, covering it for The Independent. "Ah yes," she replies, "the funeral, where all those people who had tried for years to sack him were crying, saying how great he was."

It's an early indication of what my conversation with her will be. She has a frankness, so refreshing, so rare among people who truly be called megastars, that it can occasionally take your breath away. As it does when I talk about her years at the Royal Ballet after she left Paris. There she earned the nickname Madame Non, following rows with the company's celebrated choreographer Sir Kenneth MacMillan and others. On one memorable occasion a row with Macmillan was accidentally broadcast over the loudspeaker system to the amazement of staff.

"Yes, I remember that," she says, "it was just an ego trip by him. He wanted me to dance in one of his ballets, My Brother My Sisters, and I refused. He wasn't used to that. Our relationship was a bit explosive. Maybe I was a little bit too French, adding in distinctive Guillem-ese, "I didn't take any gloves to tell him I didn't want to do it. Since that moment it was a little bit tense. Dancers are trained to be disciplined to do what they are told but I knew that time goes by and I didn't want to make the mistakes of the others. I wanted to choose. But I loved doing his Manon and Romeo and Juliet." Did the nickname Madame Non anger or amuse her? "Well, I have it as my email address."

What is certain is that, during her time there, a time dancing alongside, though never socialising with, stars like Darcey Bussell, she was a loner. "It's true I didn't mix easily. I'm not a social animal, and I had a reputation that came before me, of being very difficult, of screaming at everybody, so people tended to keep their distance."

Guillem was not just a huge draw, but, she now reveals, a catalyst for the company changing. It was she who persuaded the Royal Ballet to broaden its repertoire and do work by the innovative American choreographer William Forsythe.

"It took me 10 years to persuade them to contact and invite Forsythe," she recalls, still annoyed. "Ten years."

Her biggest ire is reserved, not for MacMillan, but the director of the Royal Ballet until this year, Monica Mason. Readers of a nervous disposition should look away now. Members of the Royal Ballet should certainly read no further. Guillem pulls no punches, certainly does not take the gloves.

On Mason her views are quite shocking: "It was exactly what a director of a company should not be, stupid, frustrated, no vision. That's why I left when she took over. She did exactly the opposite of what Rudolf did, choose people who were not at the level to be principal. "

Many in the dance world will strongly disagree with that. But what is significant is not the words she chooses, but the contrast she suggests existed between these two utter opposites wanting the world-famous dance company to go in different directions, depicting Mason as a company traditionalist, a peer of Margot Fonteyn, determined to uphold those traditions and accept change only sparingly, and Guillem a free spirit, a determined personality wanting the company to evolve and embrace outside influences. Summing up her personality and attitudes, be it on ecological or dance matters, she says: "I have a strong sense of injustice and not admitting things, just because that is the way someone says it should be. I need to understand. I need to agree."

Guillem is married to, and has been for 25 years with, photographer Gilles Tapie. They live in seclusion in the Swiss mountains, though she is insistent it is for the natural beauty rather than financial reasons. "People say it is a fiscal paradise. But London, where I lived for 18 years is much more a fiscal paradise."

It was said during her Royal Ballet years that she would only let her husband photograph her. "That is also a legend," she says wearily. "I insisted on choosing the pictures, because for a dancer this is important, you work very hard to project lines. The Royal Ballet press office [the print medium cannot really convey the look on her face as she intonated those four words] didn't want to admit this to photographers and newspapers, they didn't have the courage, and said I would only let my husband photograph me as that made an easy story for them."

Guillem grew up in the Paris suburbs, her father a mechanic, her mother a gym teacher. She was marked out as a future Olympic gymnast before being spotted by Paris Opera and Ballet at the age of 11, with Nureyev making her an "etoile" at 19. I ask if her parents came to watch her. "Yes, but after a while I had to stop my father coming. He was too emotif. He was fainting a lot. He was a very sensitive man, very fragile. It was not his world and it was difficult for him to feel at ease. He was a very nice man but very tortured." The excitement of seeing his daughter on stage, combined with a clinical condition he had, resulted in unhappy conclusions to performances. "He ended up in the emergency room, the emergency room at Paris Opera, the emergency room at La Scala."

Gymnastics did not really influence her dance, she says. "It's a completely different training. But gymnastics helped me with the spacing. I was not afraid of having my body upside down. I had knowledge of my body. Then I had to learn the discipline of ballet, which was not fun. It wasn't open-minded. The teachers were very strict and not clever. They didn't transmit the joy. I was being screamed at every day. It is the same with dance teachers everywhere. It is planetaire."

Of her own style, she says: "It is instinctive and I mix pleasure and generosity at the same time, I give what I have and what I work for as a character. I work hard to make a gift to the audience," which while lyrical does scant justice to the excitement of her performance, the eye-boggling athleticism, and what dance critic Judith Mackrell described in the case of 6000 Miles Away as "being forced to the precarious edges of movement, classical poses that implode in a rush of staccato activity, and phrases that twist and turn against the natural flow of the body."

But what of life outside of dance and outside of ecological campaigning? Was there a reason she never had children? There was, one of the more unusual I have encountered. The answer she gives is the answer of an analytical artist, talking in terms of process. "I thought about having children when I was bored in the corps de ballet, when I was very young. And the only reason I would like to have a kid is to see what is happening to a body, to create a human being, the transformation. The process would be interesting for me to live through. But a child... if he is stupid, what can you do, if he doesn't become what you want, and being responsible for someone, I'm not sure I could do that."

But her three brothers, two of them half-brothers, do have children, and so I assume she sees quite a bit of all those nephews and nieces. "No, not that much," she replies. Why? She pauses: "It's another planet." The other children of the Paris mechanic may find it hard to relate closely to the globetrotting superstar of dance, though a part of me wonders if she should try to reconcile those planets as well as wrestling with the problems of this one.

At the moment it's the problems of this one she is obsessed with, Two years ago she saw a TV programme about Sea Shepherd, and it "changed radically my life" and indeed her lifestyle. That rarity, a dancer who has never dieted, changed her diet completely. "Before I was in love with charcuterie, meat, fish, but now I have given up all of that and no dairy at all. A truly great meal is vegetables, tofu, seeds. There's more protein in lentils than in meat. I loved saucissons, really loved them, but now I make the connection between saucissons and where they come from." She talks of the big business interests opposing the ecological campaigners. "Excuse my French, but it is shitty." I would have pondered more on﻿ this if I wasn't pondering on the bizarre revelation that a French person can say 'excuse my French'.

So at 48, still attracting full houses and queues for returns, are both the dancing and the daily training regime getting harder? "It doesn't get harder or more painful because it has always been hard and painful, but after a while you get a little bit fed up. I did start to think differently when I had my first injury at 36. You listen more to what your body is telling you. Also on stage there is a conscience of fragility and it makes you different, then you pull back and when you pull back you change the timing of things and you are more aware, where before it was instinctive. Actually you start to think of stopping when you hear people say, 'so you're still dancing'."

Perhaps, after she stops, as well as devoting even more time to Sea Shepherd, she will also will give more time to another of her passions – gardening, though her good friend Sir Anthony Dowell, a former Royal Ballet head that she did and does like, said that "she loves gardening because she is burying her enemies."

Guillem tells me she will stop at 50. If that is the case then make the most of the next two years and see here whenever you can. You will find that dance can offer nothing quite like her on this troubled planet.

'6000 Miles Away', Sadler's Wells, London EC1 (www.sadlerswells.com) 20 to 25 May