It's Tuesday evening and I'm standing nervously on the corner of a street in north London outside an unfriendly-looking building. It's painted fire-engine red, has wire-meshed windows and a 'Guard Dogs, Beware!' sign looms above the door. 'Welcome to Islington Boxing Club,' grunts a deep voice from inside - but I haven't felt so out of place since I arrived at Glastonbury this year wearing a pair of flip-flops and a nightie. Why am I here?

I'm out of my depth and I want to go home. But I can't leave my friend, who has dragged me along to help her defend herself in a room full of sweaty men. So with a deep breath and pair of high heels, I push open the door. At least there is only 1 hour and 57 minutes to go.

Inside it's like a film set. A proper boxing club. There are battered bags, posters of bygone boxers and faded 1980s photographs tacked to the plasterboard. My eyes catch glimpses of proverbs such as 'Boxers do it with uppercuts' and 'Bigamy has its own penalty: two mothers-in-law'. I must remember that.

There is a moment of slight comic relief as I take in the sight of a roomful of fully grown men skipping and throwing punches in the air to the sound of Gwen Stefani's 'Hollaback Girl'. Men with six-packs - some with eight-packs - skipping in front of mirrors look quite silly, but before I've got time to gawp I'm handed a set of wrist bandages, told to hurry up, and start practising my punches.

Of the 50 people in this room, 10 of us are female. Boxing has always been considered a man's sport, but over the past two years women boxers have been fiercely promoting their position, hoping to compete in the 2012 Olympic Games. Yesterday the Women's Boxing Championships took place in Chicago, and for the first time the president of the International Olympic Committee (IOC) was there to witness the action.

Richard Baker, a spokesman for the International Amateur Boxing Association (AIBA), says: 'We are currently working closely with the IOC to introduce female boxing - it has never been in such a strong position.' The facts bear Baker out: year on year, women's tournaments are attracting more and more athletes. At last year's world championships there were 50 more contenders than competed in 2001.

It's not just female boxing that is gaining popularity. Earlier this year the BBC reported that several schools had started an initiative to bring back boxing for pupils under the supervision of the Amateur Boxing Association of England. Boxing has not been taught in schools since 1962, when a campaign to ban it from PE classes won popular support. The director of sport at Beckenham's Kelsey Park Sports College in London, Matthew Strange, believes that now there could be a place for boxing within the national curriculum. 'It is possible to see this programme developing substantially as we approach the Olympics of 2012,' he argues.

Islington Boxing Club is run by two Sicilian cousins, Enzo and Francesco. And there's no time for dawdling under their charge. 'So why did you decide to start boxing then?' asks Enzo.

'To keep fit,' I said. That was a bit of a joke: I wasn't there to keep fit - there's nothing about my level of fitness I would like to maintain. I was there to get fit.

Our instructor is succinct: 'This is no Virgin Gym or Holmes Place.' Thank goodness. Whenever I've been to the gym, I've vowed never to go back. It's not good for self-esteem. There are far too many people who already look fit. And then last year more and more gyms tried to make exercise look 'fun'. There was a flurry of new and slightly wacky workouts. Groups of girls started signing up to pole dancing and 'stiletto workouts'. At least at the boxing club there are no frills. If you want to get fit, lose weight, shape up. You have to work hard. It's simple.

Francesco and Enzo work to a tight schedule. We're taught the 'boxing stance' and three punches on our first night: jab, double jab and right hook. Hilary Swank, who played the role of an aspiring boxer in Million Dollar Baby, said: 'Boxing is the most intense workout you can ever experience. It is as if you were to take a jump rope and go as fast as you could for three minutes while someone's punching you in the stomach.' She's not wrong: so far this is certainly the most physically gruelling test I've ever faced.

Swank had Frankie Dunn (aka Clint Eastwood). My trainer is Francesco. Who you pay complete attention to. For an hour and a half, a bell rings every three minutes, which is your signal to move on to the next routine: pads, sit-ups, bags, bikes, skipping. Then more pads, sit-ups, bags - and finally sparring - where you fight against your partner in the ring. Each round, Francesco forces me to punch harder and faster: 'Double jab, double jab, right, left, left, jab.' It goes on and on. As the final bell goes, my friend and I take up our position in the ring.

As I step through the ropes I realise I am completely exhausted and totally unprepared. I remember the rules we've been taught: knees bent, chin down, fists up and elbows against my chest. Thank goodness I'm fighting my friend, a woman, and presume we have a mutual understanding not to whack each other in the head.

For three minutes we give everything we've got. We circle round the ring, trying to hit each other with our oversized gloves and anticipate each other's moves. I'm sweating so hard, all I can taste is salt and I find it impossible to find the energy to throw a punch. I think I hear Francesco shouting from the side. Is our time up? No - we've got two and a half more minutes. This is torture.

It's a far cry from the soft, dreamy tones of the yoga studio. There are no downwards dogs or sun salutations here. You get shouted at. You get made to do 50 sit-ups, 100 sit-ups, 200 sit-ups. And if you don't do them properly, you do them again. 'You can always do more than you think,' says Francesco, 'and when you are sitting at your desk tomorrow you'll be hurting.'

It's hard, but then you suddenly realise why more and more women like boxing. It's the competition. It's the fact that for two hours you can forget about everything else. It's therapeutic; your head and body are in a completely different zone. It's oddly relaxing because you can't think about anything else, you have focus on hitting and not getting hit.

Now I want to know what it feels like to really box. Other sports I've tried don't teach you to get back up once you're down. I even enjoy being told I am not working hard enough. For the last few weeks I have been constantly practising my double jabs and hooks in front of the bathroom mirror. I'll never make a professional boxer, but punching pads is extraordinarily addictive. I'd recommend it.