Scientists found that an enzyme in cows can neutralise the particles that cause porphyria.

There is a human version of the enzyme, and the researchers hope that booster shots of proteins could make it act in the same way as it does in cattle.

The research is reported in New Scientist magazine.

A royal problem

Porphyria has haunted the British monarchy since the time of Mary Queen of Scots, historians believe.

Aside from the madness associated with the disease, its best-known feature is that it turns urine purple.

It had been suggested in some quarters that sufferers were mistaken for vampires because the iron deficiency associated with the condition might make them crave blood, but, there is little scientific evidence to support this view.

Other symptoms of the rare disorder include skin and liver problems. It can also interfere with the nervous system, and this is what causes mental illness.

Porphyria is not one condition, but a group of diseases. While most of them are inherited, some can be picked up later in life.

They are caused by a build up of particles called porphyrins. These exist in all living things, but in porphyria, the enzyme that processes them breaks down. This causes illness because the porphyrins act as a toxin.

Research breakthrough

Dr Nick Jacobs at Dartmouth Medical School in New Hampshire and Dr Franck Dayan at the US Department of Agriculture's Natural Products Utilisation Research Unit in Mississippi led the new research.

Vlad Dracul - a distant relation of the UK royals - may have had the disease

They found naturally occurring enzymes that can detoxify porphyrins, but only in conjuction with two other substances called glutathione and cysteine.

The same enzymes exist in humans, and the scientists suggest that supplements of glutathione and cysteine could be given to patients with porphyria as a treatment.

"The stage is set for other researchers to feed people glutathione," Dr Dayan said.

Fascinating finding

Kate Grainger, of the British Liver Trust, said it was a "fascinating" finding.

The disease runs in Elizabeth II's family

"It's one of those little-known diseases and it's only because it attacked George III that anybody's interested in it at all," she said.

In some cases the disease damages the liver so badly that a transplant is required, she said.

"Liver diseases are sometimes the Cinderella subjects of the research world, so we're always encouraged to hear of new work that might be able to help liver patients.

"This research will draw more attention to the area, and may well act as a stepping stone to a fuller understanding of this compex organ."

She said the trust planned a £5m appeal to get more money into liver disease research.

'History might have been different'

John Röhl, a historian at Sussex University, said that an earlier treatment for the disease could have changed history.

"Because of his illness, George III did not do things he might have. He intended to rule Parliament and establish a strong monarchy.