To the untrained eye, it looks like a pretty normal loaf of wholemeal bread. It is brown, it has a crust and you can imagine it would make rather a good cheese sandwich. For Clare Smyth, however, it simply does not exude the requisite degree of doughy perfection. She casts her eye over the loaf with barely concealed disdain.

'It's not good enough,' she says, with the sort of scathing look Simon Cowell reserves for tuneless no-hopers on The X Factor. 'It's full of holes. The top's all overcooked and cracked.'

The loaf sits, defenceless and without right of reply, on the gleaming granite-topped kitchen counter. It seems to sink slightly under the weight of Smyth's disapprobation, conscious perhaps that it has failed miserably to uphold the exacting standards expected of a Gordon Ramsay Michelin-starred bread basket. 'We'll have to send it back,' she says.

The loaf is duly dispatched. The bakery sends another batch. It is still not good enough. With the bakery's third delivery, the bread is finally deemed to be up to scratch. I can't really tell the difference, but then that's probably why I'm not a head chef and why Clare Smyth has managed to land one of the most desirable jobs in the industry at the precocious age of 29.

Since January, she has been the head chef at Gordon Ramsay's eponymous restaurant on Royal Hospital Road, Chelsea. It occupies a special place in the Ramsay canon: it was his first solo venture and, since its opening in 1998, has earned him three Michelin stars and become the cornerstone of his burgeoning empire. The restaurant, which re-opened last September after a £1.5-million, nine-month refurbishment, has an eight-week waiting list for reservations.

So the role of head chef carries with it a certain sort of cast-iron prestige. Smyth is the first woman to take up the reins, under the watchful eye of executive chef Mark Askew, and one of the youngest - Ramsay likes to wind her up by telling customers that she is still 26, the age she was when she first started in his kitchen. 'I just have to stand there and smile,' Smyth says. 'He never stops.'

Still, Ramsay clearly believes that, in spite of her relative youth, she has what it takes to safeguard his considerable reputation for unstinting perfection. And that starts with the bread basket. 'For me, everything has to be perfect,' says Smyth, pinning up a loose strand of blonde hair. 'It upsets me when that isn't the case because I'm not prepared to work 18 hours a day unless it's as good as it can possibly be.

'I get here at 7am and I taste everything, all the produce that comes through the door from the mushrooms to the bread to the tomato mix for the bloody Mary. That way I know if something isn't right; it can be fixed early and I'm in control.'

On paper, this zealous perfectionism might make Smyth seem unutterably terrifying, like a sort of superannuated kitchen Dalek screaming 'Exterminate' every time a sous chef under-seasons a lobster ravioli. Yet, in person, Smyth radiates a quiet self-possession that makes itself felt without her ever having to raise her voice. She carries her authority with an extraordinary poise and calmness.

'I hate banging and shouting. It's all a bit... brasserie,' she spits out the word as if it were an undercooked Turkey Twizzler. 'In a well-run kitchen, it shouldn't have to be like that because everyone is motivated and everyone knows what they're doing. It should be like ballet.'

When the lunch service starts at 12.30, she stands at the pass, cloth in hand, eyes darting over every single plate, instantly spotting when the tiniest detail has gone awry. A sous chef is reminded to put a drop of gravy on top of the braised venison. The edge of a plate is wiped clean of minuscule specks of oil. A salad dish is sent back, wordlessly, and remade to Smyth's specifications. When a woman on table seven is too busy speaking on her mobile phone to eat the amuse-bouche pumpkin soup for over half an hour, Smyth insists that the soup is remade to ensure that it is served fresh, and at precisely the right temperature. I ask her whether the customer asked her to do this. 'No,' she replies. 'We just notice things like that.'

Smyth moves around the kitchen with a controlled elegance, her hands rubbery raw from the daily burns and splatterings, her feet moving soundlessly in a pair of battered leather clogs. The only time she swears is when one of the kitchen staff forgets to send out the accompanying sauce at the same time as the main dish and, even then, she delivers her swear words with such lethal equilibrium that you feel she's more disappointed than angry. Tall and slim, with an easy smile, bobbed blonde hair and a long, almost regal face, the effect is rather like being told off by a scarier and prettier version of Zara Phillips.

'Sometimes I have to shout,' she tells me with an apologetic smile. 'But I think that's quite good. It keeps the energy up and you have to bring people along with you.'

This unflappable demeanour surprises me. I had expected something altogether more hellfire-and-brimstone, given that she comes from the Ramsay school of expletive-laden kitchen machismo, where a light branding with a red-hot grill pan is par for the course. She first came to Royal Hospital Road on a temporary placement three years ago - Ramsay was so impressed that he took her on as a permanent demi chef de partie and she has been there ever since, apart from two stints abroad, training under Alain Ducasse and Thomas Keller. But the extraordinary thing about Smyth is that, despite her age, she seems to have complete assurance about what she is doing, without any arrogance about what she has achieved.

'Clare has an instinctive palate and a self-determined level of focus, a tunnel vision for perfection, that is very rare in any chef today - male or female,' says Ramsay of his newest protegée. 'She has a level of composure, a posture, that is intimidating, almost like a boxer entering the ring and she dresses food like Picasso.

'I would say that a talent like Clare Smyth comes through the kitchen maybe once every 10 years. The last time was with Angela Hartnett in Aubergine and that was back in 1995.'

Ramsay says that there is 'an articulation of calmness' about female chefs that makes them particularly impressive when they get it right. He has been so taken with the wealth of up-and-coming women chefs that next year he says he aims to have an entirely female brigade in the Royal Hospital Road kitchen.

'A lot of chefs would despise that idea,' he says, relishing the foretaste of controversy. 'I couldn't think of a better person to spearhead that campaign than Clare Smyth. She's a phenomenon.'

At 4.30pm, after the last diner has ambled out into the street, filled to the brim with oyster velouté and a sense of all being right with the world, Smyth unties her black apron and sits down for the first time since early morning. She has about an hour to spare before the never-ending preparation begins for another 40 covers this evening. On an average day, she will scooter home to her flat in south London at about 1am. She gets five hours' sleep a night.

'You do need stamina for this job,' she says, 'but I'm completely passionate about it. Ordinary life seems a bit boring by comparison.'

It helps, of course, that Smyth has always wanted to be a chef and is prepared to make enormous sacrifices to fulfil her ambitions. She grew up on a farm in Northern Ireland, halfway between the Bushmills whiskey distillery and the Giant's Causeway on the Antrim coast. Her father, William, is a horse trainer and her mother, Doreen, worked in front of house at a local restaurant. The youngest of three, she remembers being 'a very competitive' child.

'I think that was partly to do with being the youngest, with trying to make myself heard. My brother [Darren] and sister [Sharon] are four and five years older than me and my brother and I used to ride ponies together and try to beat each other at show-jumping. I was always in competition with them.

'My father was someone who drove you hard. It wasn't good enough to come second. Even if you won, it wasn't good enough because you'd made mistakes. So, yes, I'd say that's where I get my perfectionism from.'

No wonder she has thrived under Ramsay. Is it exhausting living up to such unattainable standards? 'Yes. But I feel that the more I learn, the more I have to do. Anything less would be an injustice to myself. I feel very strongly about that.'

It was a weekend job as a teenager at a local restaurant that first got Smyth interested in cooking. To begin with, she dismissed all notions of becoming a chef in favour of 'a decent job, like a lawyer or a doctor' but, as time went on, she found herself becoming more and more enamoured by the lure of the kitchen.

'I started reading cookery books written by chefs like Anton Mosimann and gradually I just decided I wanted to become like them. Northern Ireland does not have a great restaurant scene so, at the age of 16, I left home and moved to England.'

She says this with such blithe nonchalance that she could almost be listing the ingredients of a ham-and-cheese omelette. With a little probing, it transpires that Smyth more or less ran away from home against the wishes of her parents.

'My parents were furious with me,' she admits. 'But I was so determined. I finished my GCSEs, I saved up all my money from my part-time job, I got myself an apprenticeship in Surrey at Grayshott Hall [spa hotel] which provided accommodation and I found a course at a college outside Portsmouth and enrolled in it. I organised it all myself. I remember a friend at school saying, "You can't just go and do that", but I did. I was quite headstrong. I knew exactly what I wanted.

'I was meant to be going on a two-week holiday. At the end, I called up my parents and said, "You know what? I'm not coming back". I was quite hard about it because I knew I wouldn't have been able to do it in Northern Ireland.'

At 16, Smyth found herself in a strange country, without friends or family around her, holding down a full-time job and living among strangers, most of whom were much older. 'It was really difficult to be by myself. I lost my accent within two weeks and I think it was because I was so young and I was desperate to fit in. My mother couldn't recognise me on the phone. I remember feeling lonely. It was hard.'

The accent went permanently, to be replaced by a curious transatlantic mixture of rapid-fire vowels that sounds almost South African. How do her parents feel about her chosen profession now? 'It's taken them a long time to be supportive,' she says. 'They don't understand the industry, they're not foodie people and they can't understand why I work these hours or travel as much as I do. They think I'm crazy, but I think that they're proud of me, although they haven't got round to eating in the restaurant yet.'

After gaining her NVQs at Highbury College, Portsmouth, Smyth found a job as an 18-year-old commis chef at Bibendum in London. A year later, she went to work as sous chef at St Enodoc Hotel in Cornwall, eventually becoming the head chef after a six-month stint working for a party-catering company in Sydney. She spent her holidays doing 'stages' with chefs that she admired, racking up experience with Michael Caines, Heston Blumenthal and the Roux brothers before walking into the kitchen at Gordon Ramsay.

'When I first started here, it was very, very male-dominated,' she says. 'There was a hell of a lot of testosterone in that kitchen. I was told I wouldn't last a week - Gordon even said that. There were people saying, "It's not for girls, you shouldn't be here". It took me a long time to earn respect. I had to work twice as hard. I could never say I was tired or I'm sick or I've cut my finger because the response would be, "It's because you're a girl". But actually, the guys are the first ones to whinge when they're tired or they've got a sore thumb.'

Smyth has taken one half-day off sick in her 13-year career - and that was when Ramsay himself ordered her home because she was 'looking green and throwing up'. At Bibendum, she pierced her hand with an oyster knife and had to be taken to A&E against her will, which hardly counts as a duvet day. Her determination is such that she also managed to talk herself into a trainee placement at Alain Ducasse's world-renowned Louis XV restaurant in Monte Carlo - on sabbatical from Gordon Ramsay - despite being unable to speak a word of French.

'It was a big deal because if you're working for another chef, they think you're going to steal their ideas,' she says. 'Plus I was English and I was female, so it just didn't go down too well. I promised the chef I'd learn to speak French and I paid for a four-week intensive language school in Villefranche. Then they said yes. It was phenomenal. Amazing. The Louis XV is so old-fashioned, so classical and that's what I really wanted to learn.'

Given her monumental drive and focus, it seems unlikely that Smyth will ever feel she has achieved all that she aspires to. She seems genetically programmed never to be entirely satisfied, always wanting to strive for more.

'I don't have a boyfriend but I would absolutely like to settle down eventually. I think it's the reason I push myself fast because I figure that I have another, maybe, five years at the top. Then I can have a family. As a woman, you have to achieve it younger. Men have all the time in the world.

'I used to want to have my own restaurant, but it's such a fickle industry that there's no guarantee and some of the best chefs fail.'

Maybe. But I doubt very much that Smyth will be one of them. As she walks back to prepare for the dinner service, a woman sitting by the bar asks her to sign a newly purchased Gordon Ramsay cookbook. Smyth agrees politely but, as she waits for the waitress to bring her a pen, she jiggles excitedly from foot to foot. It is clear that she wants to head straight back into the kitchen. And the bread better be good when she gets there.

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· Royal Hospital Road, London, 020 7352 4441.