Chocolate, says Mel Cornwall, can effectively lighten the load of a tough day in the classroom.

Any chocolate will do, as long as it is close to hand. The 34-year-old trainee teacher always kept a large box in the fridge, in case the need to nibble should suddenly strike. “Chocolate calmed me down if I was feeling stressed,” she said. “Studying is very demanding, so I felt I had an excuse.”

Ms Cornwall is to feature in Fit Street, a new Channel 4 series that attempts to increase the life expectancy of four south London residents, by persuading them to give up chocolate, cigarettes and alcohol.

At 14st 8lbs, and only 5ft 3in, she has long battled with her weight and knew that surrendering her particular indulgence would not be easy.

“I’ve taken diet pills,” the mother-of-three said. “I’ve joined WeightWatchers about 10 times, then given up after two weeks. And I would take the bus to university and tell myself that I was exercising, because I walked to the bus stop.”

For the series, she teamed up with a range of fitness and nutrition experts, including Adam Carey, nutritionist for the England rugby team.

Dr Carey immediately classed her as morbidly obese. He then informed her that in four weeks’ time she would take part in a 15km bike ride.

“I laughed, because I thought he was joking,” she said. “Then I thought, I’ll say yes, but that doesn’t mean I actually have to do it.”

But it did mean that she was committed to a harsh daily regime of lunges, squats, swims and long walks in the early morning. And she was forced to relinquish her beloved chocolates, instead relying on a diet of yoghurt, fruit and vegetables.

“For the first two weeks, I really craved chocolate,” she said. “They had to send me to a hypnotherapist. But now I’m discovering fruit and salad. I used to feel guilty when I ate, and tell myself I was a fat cow. But eating fruit makes me feel good about myself. It’s changed my attitude to food.”

With glowing skin and looser clothes, she also began to seriously contemplate Dr Carey’s bike-ride challenge.

Eventually, she completed the ride in 38 minutes: “I was knackered. I was just numb. But, once I’d had a shower, I was on a high. I thought, what’s next? I’m going to Jamaica at Christmas, so maybe I’ll aim to wear a bikini.”

And, she said, the thrill of losing weight may well replace chocolate as support for a tough day in the classroom: “Maybe I could take healthy-eating classes, or we could grow our own vegetables.

“It should be an inspiration for the kids. I thought getting fit was impossible. Now I realise nothing is impossible.”

Fit Street will be screened from August 30 to September 3