At stake: the reputation of a French heroine, after expert dismisses Joan of Arc's story as a royal fable

When the French authorities called upon Serhiy Horbenko to throw fresh light on the country's medieval heritage they never anticipated that the Ukrainian orthopaedic surgeon would attempt to undermine the most potent patriotic story in the nation's history.

But Dr Horbenko, who has established an extraordinary reputation for his expertise in examining skeletons, has risked Gallic ire by casting aspersions on the accepted story of the demise of St Joan of Arc.

The death of the teenage warrior burnt at the stake as a witch after a trial prosecuted by her English enemy and their allies in the Catholic Church, is one of the defining moments in the French national psyche.

But Dr Horbenko's research into the skulls and skeletons of France's long-dead royals has led him to conclude that the woman on the pyre was not Joan at all but another French noblewoman. The woman known as "Joan", he says, lived on for decades after her supposed execution.

The surgeon invited by the French authorities to study the skulls of the French King Louis XI and his wife, has suggested that with the English armies threatening the French throne, the monarchy needed a miracle and their supporters concocted one.

He said: "I believe that a group of nobles thought up the plan, in a time when people were deeply religious and believed in miracles, to influence the French people and armies and to demoralise the English. They wanted a woman sent by God to defend France and to legitimise the Dauphin's claim to the throne."

He said that the person who was chosen to play the role of saviour - always ascribed to Joan - was in fact a noblewoman called Marguerite de Valois, the illegitimate daughter of the previous monarch Charles VI.

The life of Joan of Arc has been the subject of fierce debate for centuries but is also one of the most well-documented in early modern history. According to most historians, she was born on 6 January 1412 in the village of Domremy in what is now Lorraine in eastern France.

Three years later, Henry V invaded France in pursuit of his claim to the French throne and won an emphatic victory at Agincourt. In the ensuing period, as English armies established a stranglehold on northern France, the young Joan is said to have heard voices from God, telling her to go to the aid of her king.

When she took up the call to go to the siege of Orleans she was still only 17 years old. After an audience with the Dauphin, who ultimately became Charles VII, and interrogation from theologians in Poitiers she was placed at the head of the French army as a "Saint Catherine come down to earth". In little more than week, the siege of Orleans had been lifted and, during the summer, the English were driven from the Loire valley and Charles the Dauphin, thanks to the actions of Joan, was crowned King at Reims cathedral.

Dr Horbenko, believes that Marguerite de Valois was in fact the illegitimate daughter of Charles VI and, in possession of fine military skills, performed her role much better than anyone expected. She became such a powerful figure in the eyes of her followers that she was herself perceived as a threat to the French throne.

"I think that if she had revealed her Valois lineage, she could have secured the backing of enough nobles and soldiers to overthrow the Dauphin," he said. Dr Horbenko believes that Marguerite was removed from the scene and another woman was substituted top become the martyr.

According to the standard version of the story, the relationship between Joan and the Dauphin became strained but she continued to lead the army until, in the following May, she was captured at Compiegne by Burgundian forces allied to the English.

Mr Horbenko accepted that his theory of a substitute Joan might seem "incredible to modern day people who have cameras and video recorders and are used to instant news and images of famous people on television and in newspapers and magazines". But, he said: "None of those things existed then and most of those who saw the military leader, Joan, did not see her when she was taken captive by the English."

His controversial thesis emerged after he was invited to France to carry out research into the background of St Bernard. He had built a reputation for his expertise in using the bones of historical figures, such as a medieval Ukrainian monarch and a 5,000 year-old Scythian tribal leader, to reconstruct their appearances.

After the St Bernard project, Dr Horbenko was invited by the French authorities to put faces on the skulls of Louis XI and his wife, work which led him to the Basilica of Notre Dame de Cléry near Orléans.

He asked for permission to open tombs elsewhere in the Basilica, which was the last resting place of members of France's royal Valois lineage. "As I opened up the tombs I started to come across information that led to a conclusion I could hardly believe myself," he said.

One skeleton, in particular, shocked him. "The bones indicate that the woman wore heavy armour and had developed muscles that I have seen in other fighters of the age. For instance to ride a war horse took special kinds of skills and training which you can detect from the remains if you have enough experience," he said.

"Each skeleton is as distinctive as a fingerprint. Each bears signs of wear or disease that allow you to match them up. You can establish family relations using skeletons with a fantastic degree of accuracy."

Dr Horbenko said: "Charles VI ... was worried for [Marguerite's] safety and I believe that from an early age he discreetly trained her in military skills, perhaps so she could better defend herself."

In the history books read by every French school-child, the captured Joan was sold by the Burgundians to the English in 1430. She was held in a secular jail, where she insisted on wearing the trousers and tunic she had worn into battle as of protection against being raped.

The English knew that by having Joan condemned by the church authorities they could discredit the French King. After being put on trial and convicted of being a witch and a heretic she was burned at the stake in Rouen market square on 30 May 1431.

Dr Horbenko believes there may have even been a further switch so that the place of the woman who made such an impression at the trial was taken by one of five women he learned had been condemned to be burned to death for witchcraft.

The surgeon said that Marguerite, meanwhile, was effectively held as a prisoner for the remainder of her life and died in her late fifties. He is convinced it is her remains interred with those of Louis XI, the Dauphin's son, as a sign by those who knew the secret that she had preserved the throne of France.

The theory has not gone down well with the authorities who invited Dr Horbenko in. Denise Reynaud, the deputy mayor at Cléry who commissioned Dr Horbenko described the Ukrainian as a "very difficult man to work with owing to his Slavic temparment".

Olivier Reffier, a senior official within the French Culture Ministry, also takes issue with Horbenko's theory, saying it was nothing more than speculation. He said that the bones now in the Basilica have undergone so many 'peregrinations' that it is possible they do not even belong to the Valois family.

The Ukrainian's claims will not be well-received and Dr Horbenko knows it. "Many people revere Joan of Arc and I do not take lightly the implications of shattering this myth," he said.

"So far there has been little publicity given to the work or theory. I know that it must be thoroughly checked because this is an important part of French history, a myth that has sustained them for centuries."