She bounces in, light as a feather, perching on the balls of her feet (or patent-leather slip-ons, as it were) and sits down gingerly on the chair in front me. If anyone ever doubted her youth, as many do, seeing her airy, childlike entrance would put any doubt to rest. Claire Fitzwilliam is freshly eighteen and looks it – even looks, perhaps, a bit younger.

“I like to move,” she says, in fact shifting the way she’s seated herself already. “Comes with the career. You’ll never keep us still.”

She’s set to become the Royal Ballet’s youngest principal – ever. And perhaps some of the excitement about her promotion has fed her nervous energy. Or perhaps it truly is a sign of what a remarkable dancer she is.

There has, of course, been plenty of commotion and scepticism over the recent decision. Some argue that she is far too young to be engaged in the pressures that the company puts on their tip-top career spot – “You have to take every lead role in every show,” Fitzwilliam tells me – and some argue that the fact she only just graduated from the Royal Ballet School means her skills could quite possibly be lacking.

Most people would argue the second point with a fervour reserved only for the likes of Margot Fonteyn and Anna Pavlova, those household ballerinas that even the most sheltered of people could recognise in a pile of names.

“I’ve danced all my life. It’s all I remember focusing on as a child. There were a lot of political problems at home and my mother, who is English, wanted us to be focused on something that is good – inherently good, entertainment. And she was a dancer herself so she put us in lessons. I wanted to be a Broadway star for awhile.

“My sisters didn’t take, at least not for long, but I loved it.”

Fitzwilliam has a host of awards under her alignment belt (which, she tells me, she still uses from time to time), including the illustrious Prix de Lausanne, which gave her a scholarship she didn’t really need – so they figured out a way to donate it back.

“I only did the competition to test my skills. It was my second year at the school and I had come out of a very difficult time and [a teacher] suggested I give it a go, so I did. And I won! I was very surprised and very honoured. Incredibly honoured,” she says softly, delicate fingers adjusting the hem of her dress.

The difficult time Fitzwilliam is alluding to was the fall into an eating disorder. She refused to eat, or found ways to hide that she wasn’t eating, for several months after starting at RBS.

“My sister wasn’t speaking to me, as she was angry we had to move. A week after I started school, I was told that I was a late start – and I was, at thirteen, but I had already been dancing for nine years. But that comment made me feel as though I had no future and to make matters worse, I started growing up and out and I was terribly afraid of losing the look.”

What look? Fitzwilliam is still a little on the thin side. Her shoulder blades shift under fair skin, and though her legs are long and muscular, that seems to be all they are. “You have to watch yourself, not just your weight, but your overall health. If you have a bad diet, you will fail. Overworking yourself is very popular in dancing now and I am quite guilty of it, myself, and you have to be fit in order to keep from keeling over after every practice.”

But she acknowledges that there is a line and that the line is often crossed. “The pressure can confuse any dancer, even the best. Ballet is a fickle thing – or not even fickle, really; picky. But it’s still quite daunting to keep up with it and there are strict rules and confines which are difficult to move within unless you fit.”

Rest assured, she fits. Fitzwilliam became a member of the corps de ballet in 1982, when she was fifteen. It was her performance in the final school show that captured the attention of Sir Kenneth MacMillan and a host of other balletomanes and influential people. She was an apprentice by the end of the day and a member of the corps in less than a month after.

The reasoning, she recently found out, was that she hadn’t left school yet and it would be impractical for her to be a soloist or principal until she was somewhat older, taller – an adult. At the time, however, she couldn’t wait to become one of the four swans, and then Odette herself.

“I pushed myself over the edge during my first year in the company, to be certain. That year was the first year I sustained a dance-related injury that actually kept me off my feet. I sprained my ankle and became quite depressed and it was just a mess of a thing.”

That autumn, however, she was offered places in both the American Ballet Theatre and New York City Ballet. Mikhail Baryshnikov being her dance idol, to be a member of ABT (and a soloist, as well) was the top choice, but she was denied. “My parents said I couldn’t live in a strange city on my own (we had only been in the states twice, and never to New York City) and they couldn’t move. I got over it fairly quickly.”

She’s since met Baryshnikov, whom she refers to as Misha. “I danced with him before a show. I could have died.”

When asked if she considers herself to be at his level yet, as they are both principals in world-class companies, she quips, “Ask me again in twenty or so years.”

Many do, however, consider her to be at the level of the greats already. Despite her age, she is highly gifted. In ballet, some attributes of great ability are good technique and better stage presence. She has both – but her technique is far better than good. In the ballet world, her most prized features are her extremely high extension (she can make her legs very long and straight, in very layman’s terms), strong, arched feet, a long neck, gentle arms and strong movements.

I don’t consider myself an expert, but when she danced, I could easily see why she has been called the next Margot Fonteyn, why she has danced with Baryshnikov, and why she is the youngest principal in Royal Ballet history. She is young and tall and strong, even in bare feet. And the dancing is a passion that is easily-readable in every movement.

When asked if she wants to dance for the rest of her life, she says, “Oh definitely. They’ll be surprised, I’m certain.” A cheeky grin.

And what of ABT? “If they offered me a spot again in a couple of years, I’d love to go dance with them. But being principal here means I have a loyalty to this company and I’d probably like to be a guest artist.”

She pauses, making a face, concealing a thought. “But if [Mikhail Baryshnikov] asked me directly, I’d certainly go.”

Even the greats can have greater idols.

“Ballet is what I’ve always done. It’s who I am. I define myself by my dancing. I get a lot of comments from friends and family—they tell me to slow down, they tell me not to ruin myself or burn out, but I burnt out quite young.”

Before she was a professional, as she was winning prestigious international competitions, her eating disorder nearly claimed her dancing. “I was hospitalised,” she admits, looking uncomfortable. “Two weeks of intensive therapy and a month of out-patient treatment after that. I was still dancing during all of this. I pitched a remarkable fit that scared my therapists into submission.”

Does the paranoia go away? “No, it doesn’t. If there is one thing I hate about the sport it’s that pickiness. That rigidity. I remember quite vividly one of my teachers grabbing the loose skin of my friend’s back as she bent back at the barre and telling her in no uncertain terms to stop drinking milk immediately. I was shocked. She came to me next and told me I was getting tall and to watch my diet.”

Outrageous though it may seem to allow teacher to point out physical flaws, this is a common ballet trait that has lasted centuries. “It’s not ballet if you’re not being beaten into form. I hardly condone that sort of thing and I’d never tell a friend she looked round about the middle. I expected comments. But expecting them didn’t make them any easier to receive.”

It is difficult to believe that a girl praised and loved by the ballet community has been critiqued negatively at all. She is a beautiful girl with fair, fine features and a prominent family. Secondary to Claire’s position in the company is her trust fund and connection to the Fitzwilliam Diamond Company. Her father, the CEO, has supported his daughter’s decision not to join him as a jeweller, but she has not escaped comments that her ability to move from one position to the other without much waiting was fuelled by frequent donations to Covent Garden by the family.

“Oh, they’ve donated to the arts for decades,” Claire tells us, looking slightly uncomfortable. “My grandmother loved the ballet and was on vaudeville in America back in the 1920s and 1930s.” Vaudeville often featured chorus lines of glittering women, dancing in time.

“I do dislike it when people ask if my father bought me my position, or if my aunt had anything to do with it.” Her aunt, Marion Andrews, danced with Kirov Ballet in the USSR for decades, and knew Baryshnikov before the rest of the world did.

“I feel it belittles everything I’ve worked for. Do they watch the shows I’m in? Do they see the reviews? I’ve not yet seen a show panned for poor dancing. When it is, I will step down. But I believe that I can dance and I dance because it makes me happy, not because I have money.”

Claire, though young, has the sense of someone twice her age, and twice the dedication. “I work constantly. It’s all I do. I just left school this past June so I’ve got a brand new schedule to adjust to. At first I was terrified!”

Terrified of leaving school, I ask. She looks at me in an embarrassed way. “Terrified I wasn’t going to be dancing nearly enough. Before we had classes every day, technique, pointe, pas de deux, academic lessons like anatomy and French and weekly experiences doing other dance forms, like modern and jazz and even gymnastics! But I figured out a way. I’m dancing four hours a day, six days a week, and on Sundays I dance for just as long, but on my own. I am constantly critiquing myself, wanting to be better, wanting to be the best, I won’t lie. Every dancer wants to be the best.”

And no doubt, she will be.