Natasha Demkina has a lot of explaining to do. Sat with her mother and younger sister in a bright London office, the 17-year-old from Saransk, a rundown town 400 miles east of Moscow, seems the typical teenager. But less typically, Natasha claims she possesses an unusual gift. She can, she says, look deep inside people's bodies, watch their organs at work and spot when things are going wrong.

That the finest medical minds in the country are not queueing up to examine Natasha is no surprise. To all but a vanishingly small number of the medical community, her claims are too ludicrous even to warrant discussion. Others suspect it is nothing more than a trick, a medical variant of a fortune teller.

But such concerns rarely carry much weight in the media. In January, the Sun brought Natasha to Britain, prompting a flurry of credulous stories about the "girl with the x-ray eyes". During her visit, she went on ITV's This Morning and impressed Fern Britton by spotting her sore ankle. She also managed to unnerve the show's resident doctor, Chris Steele, who when told he might have something wrong with his stomach, pancreas, liver and kidneys nipped off for a scan at a Harley Street clinic. He was later given the all-clear.

While the media happily courted Demkina, the dismissive attitude of the sceptics seems to have vexed her. Earlier in the year she agreed to take part in a scientific test to prove once and for all whether she can really do what she claims. "I just wanted people to acknowledge that this phenomenon exists," she says.

Natasha says she first became aware of her "gift" when she was 10. "I was at home with my mother and suddenly I had a vision. I could see inside my mother's body and I started telling her about the organs I could see. Now, I have to switch from my regular vision to what I call medical vision. For a fraction of a second, I see a colourful picture inside the person and then I start to analyse it," she says.

As word of Natasha's claims spread throughout Saransk and across Russia, people started turning up at her door, mostly those whom local doctors were at a loss to diagnose.

Natasha is in London for a publicity tour, courtesy of Discovery Channel, who filmed the experiment for a programme to be aired at 9pm on Thursday evening. The experiment was designed by scientists working for Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP) an organisation of professional sceptics who have previously tested, and failed, the likes of Uri Geller.

For the first part of the test, Natasha was asked to try and diagnose the conditions of people who had described specific ailments to the scientists. At the end of the consultation period, most were impressed, feeling that Natasha had pinpointed their conditions.

In the second part of the test, Natasha was sat in front of seven people and given a list of medical conditions ranging from an artificial hip to a metal plate that had been implanted in one of their heads after the removal of a tumour. The scientists decided that if Natasha could correctly identify who had which operation five times out of seven, she would pass.

After four hours, Natasha had made her final judgment and the scientists convened to announce the results. Though she correctly identified four of the patients, she misdiagnosed three, including one who had a missing appendix. She had failed, the scientists declared. Natasha now says she was unhappy with how the test was carried out. "The atmosphere of the testing was unfriendly. The conditions I was looking for were in some cases dubious. Why is that if I get five out of seven I pass, but if I get four, I'm a total failure?" she says.

But the scientists who designed the experiment say a failure is a failure. So what is their explanation of what is going on?

"At best, she's done this a lot and she has a real expertise at being able to look at people and make reasonably accurate diagnoses. But at worst, there's something else going on," says Richard Wiseman, a professional magician turned psychologist and paranormal investigator at the University of Hertfordshire.

Prof Wiseman, who helped design the test, says that although they have no proof Natasha cheated, a lot of text messages were being sent between her and her companions during the test, something the scientists had expressly forbidden.

"I wasn't convinced there wasn't other stuff going on," says Prof Wiseman. Why so many of those who go to see Natasha believe in her ability to diagnose their ills is akin to people believing in fortune tellers, says Prof Wiseman.

"When I saw her do her usual readings, I couldn't believe the discrepancy between what I was hearing and how impressed the individuals were.

"I thought they were going to walk away saying it was embarrassing, but time and again, they said it was amazing. Before each reading, I asked the people what was the main medical problem and Natasha never got one of those right," he says.

Prof Wiseman believes people listen to Natasha's comments and focus only on those aspects they believe. Regardless of whether Natasha is believed by the scientists or anyone else, if she becomes more famous for her claims, she could stand to make a lot of money. While living in Saransk, she began charging 150 roubles (£3) for a consultation, later upping the fee to 400 roubles. In a town where the average wage is £15 a week, the money could quickly become useful.

Earlier in the year, Natasha explained the fee was a means of funding her through medical college in Moscow. She started earlier this month, though the state has paid her fees. Her family has moved to Moscow. "If they are in it for the money, I think it's more about potential earnings than what she's earned so far. If you think of the number of ill people in the world who don't know what's wrong with them, that's a huge, huge market and it could become big business," says Prof Wiseman.

Whatever becomes of Natasha in the future, she is adamant she is not motivated by fame and fortune.

"I don't think about how lucrative it could be. I just want people to accept me. The dream is, if I preserve my gift, to use it but on the basis of proper medical knowledge."

· The girl with the x-ray eyes, Discovery Channel, 9pm, Thursday September 30