BBC 1's QED - which brought together the world's top fire experts - looked at cases of spontaneous human combustion from around the world.

Spontaneous human combustion hits the headlines

And the programme discovered that the so-called wick-effect, in which a body is devoured by flames from its own body fat, is behind the mystery.

Using a dead pig wrapped in cloth, they simulated a human body being burned over a long period and the charred effect was the same as in so-called spontaneous human combustion.

The QED team spoke to Paul Haggarty who as a young fire officer was called to investigate a fatal fire in Pennsylvania. The victim was an elderly widow and what he saw still haunts him.

Paul Haggarty: "There is no way you could explain it"

The victim was consumed by fire except for the legs. Even a wooden table nearby was untouched.

Mr Haggarty said: "There is no way you could explain it. None of the firemen had seen anything like it."

The most recent recorded case in the UK happened in 1992 in northern England.

Again, the woman's legs were left untouched, while the rest of her body was burned to a cinder.

Home Office pathologist Mike Green, who examined the body, does not believe in spontaneous human combustion.

But he said: "The first time you see it you rock back on your heels and your ask yourself 'How is it that the body has been totally consumed and nothing else has been damaged?'

"It beggars belief."

Even cremation cannot create the same effect

Explanations include balls of lightning or a build up of methane inside the gut.

But forensic scientists are now convinced that the cause of such incidents is the so-called wick effect, started by a source as simple as a cigarette.

This means that in certain rare circumstances a human being can burn like a candle.

But the fact that human bones were completely destroyed remained a mystery, until now.

Even in human crematoria where temperatures are 700-1000 centigrade, the bones are not destroyed.

Dr John de Haan of the California Criminalistic Institute used a dead pig in a gruesome experiment to show that small flames can consume a human being with the help of burning body fat.

The pig is prepared for the experiment

A pig was used because it closely resembles a human's fat content.

The pig was wrapped in a blanket and a small amount of petrol was poured on it.

After five hours of continuous burning the bones were being destroyed.

Dr De Haan said: "The sort of damage here is exactly the same as that from supposed spontaneous human combustion."

Pig's bones were destroyed after five hours

The scientists believe they demonstrated how a case of spontaneous human combustion can occur through normal processes on a person who has been knocked unconscious.

The wick effect means that a person could burn slowly without attracting attention from passers-by.

It also explains why only part of the body, the part which is rich in fat, burns while the rest stays intact.