London's hottest doctor on sexual health, size zero and David Cameron

Not only is he a proper London GP (he works two days a week at Doctorcall on Harley Street), he is a major TV property.

Last week he won a Bafta for Embarrassing Bodies online (the interactive website gets huge traffic after every show, especially from the 16-34 age group). "I'm very proud of it. But I never expected to win. We were up against the Olympics!" he says.

It's Jessen's steady presence that stops the show's reality format turning into car-crash TV. He wants to break painful taboos about the body but ensures none of the participants is ever bullied or humiliated.

He talks to teenagers in their own language without white-coat formality but doesn't let them off the hook if they are damaging their bodies (whether from unsafe sex or an addiction to sun beds) - he tells it like it is.

Some of the footage on last year's Embarrassing Teenage Bodies was eye-watering, but Jessen is unrepentant. British teens have the worst sexual health in western Europe, so the show needed to be accessible.

"We have to get people to watch it, but while they are being shocked or fascinated, they're learning something," he says.

Jessen is very game - he presented one episode of Embarrassing Teenage Bodies on the beach in his swimsuit - but he is careful about his persona.

"Channel 4 is wonderful but the executives always want to push you a bit further - you need to remember that when all the TV work falls apart, you are still a doctor. I need to have professional integrity. I can't be a clown camping it up because I would lose my authority."

Last month, Jessen hit the headlines when Gordon Brown's spin doctor, Damian McBride, sent an email to Labour blogger Derek Draper, suggesting they spread gossip - entirely unfounded - that David Cameron may have suffered from a sexually transmitted disease.

He also suggested "inserting [a] picture of Dr Christian Jessen", implying he had treated the Conservative leader.

Jessen can't discuss Cameron (it would be a breach of medical ethics). "I can't say I've never met him," he says carefully. But nor can he deny the rumour. "It's interesting, because if you say, 'No, someone's not a patient', is that also a breach of confidentiality?"

But he is indignant about politicians indulging in puerile humour - and mocking the idea of sexual health.

"I was particularly disappointed because there am I slogging away trying to make people feel more comfortable talking about these things, and then this idiot undoes it all in one email by implying it is shameful and embarrassing. It made me very cross."

Jessen, still only 31, has packed a lot in. He graduated from UCL in 2000 with an MSc in general medicine, infectious diseases, travel medicine and sexual health/HIV.

He lived for a time in Kenya and Uganda where he researched HIV and malaria in children, and is a columnist for several magazines, including Gay Times.

Undeniably posh - he went to Uppingham School in Rutland and is a member of the Garrick Club and the Royal Institution - he's also charming and self-deprecating.

Jessen was born and grew up in London. His father is a physicist and chemical engineer; his mother a translator. At school, he wanted to be a musician and opera director, but proved very good at science: "They told me you can always go back to music later!"

At college he realised he wasn't cut out to be a hospital consultant. "I loved seeing my patients but I hated the very old-school jumping through hoops that medicine entailed. I think that's why I took an alternative path."

He went to Africa where he encountered the full "calamity" of HIV. "It was the best thing I ever did. I was an only child from a sheltered public school and it made me grow up," he says. Returning to London, he decided to specialise in sexual health.

He started doing morning TV, then was offered Embarrassing Illnesses. He looked after Jack Osbourne through his travels in India for Finding God, and appeared on the documentary Harley Street.

Supersize vs Superskinny put Jessen on the map. On the show he helps an overweight and an underweight person swap diets for a week to tackle their chaotic eating patterns.

"It's a brilliant concept to make them sit and watch someone else trying to exist for seven days on what they eat themselves. By the end of the week they're apologising to each other, and that's why it worked."

He hates the whole concept of size zero. "I think women are under an awful lot of pressure but they need to realise an utterly lean fashion body isn't a body for bringing up children - if you get ill or pregnant you've got no reserves at all."

Jessen is driven. He works with health charities, plays the oboe, rides horses and models. It would sound comical if he wasn't so nice.

Being a doctor, he's not shouting about his personal life. "I'm in a relationship. But I'm away so much travelling it's an issue that needs to be addressed. But then I've always been in slightly uneven relationships in that respect. I think I'm slightly bossy, I like to be Queen Bee."

His parents are fine about his explicit TV shows. "I'm an only child so they have to rub along about what I do!" In fact, his retired mother, who now teaches English in Wandsworth Prison, says it helps her bond with her prisoners.

He never wants to stop seeing patients. "People are frightened, people are fragile. As doctors, we've always regarded health as our little secret. You come and see us in our inner sanctums and we're the ones in control. The traditional doctor was a paternalistic figure who said: 'Take the blue pills once a day and off you go.' That needs to change."

Jessen is an advocate of the cervical cancer vaccine. "It does not encourage promiscuity and underage sex in girls who have it, and parents who withhold it from their daughters are being reckless. Would they do the same if there was a chlamydia or even an HIV vaccine? I certainly hope not."

But he also believes in tough love. "People need to take more control of their health. I get frustrated by patients who have absolutely no idea about a condition they have. Why aren't they interested in why their bodies are going wrong?"

He agrees doctors should extend surgery hours in London. "I think life is changing and we have to adjust. How does a City boy or girl get to a doctor?" But he thinks there should be a charge (excluding people with chronic conditions) to generate income and make sure it's a genuine appointment. "I'd charge a fiver for a GP appointment for everybody. I think it would stop a lot of the rubbish."

Next, he starts filming a new show, The Ugly Face of Beauty. "It's going to be a campaigning show about people having plastic surgery abroad and the NHS having to pick up the pieces when it goes wrong."

And he's going on tour around the country in September with An Audience with Dr Christian.

He laughs when I say he's been typecast as the doctor who talks about willies on TV. But the McBride affair has taken him into another league.

"Secretly, on that Sunday [when the story broke] I realised I couldn't pay for PR like that. I thought: 'Wow, this is great, every MP in the country will know who I am!'"

Embarrassing Bodies is on Channel 4 on Wednesdays at 8pm.