Richard III may have had an ignominious resting place under a Leicester car park, but spare a thought for Henri IV. First the French monarch was disinterred from the royal sepulchre by revolutionaries and thrown into a mass grave. Then his head was cut off and – allegedly – turned up in the attic of a retired tax inspector.

Worse, while British experts have confirmed that the deformed skeleton found in Leicester is “almost certainly” that of Richard, bearing signs of fatal wounds he suffered during the battle of Bosworth, French scientists are still fighting over the disputed remains of Henri, who was assassinated in 1610 by a Catholic fundamentalist.

Unlike Richard III, who was reviled during his lifetime, “good King Henri” was credited with kindliness and seen as a potent symbol of national unity and reconciliation. Baptised a Catholic but raised a Calvinist, he ended bitter religious wars in France and took pains to ease the daily travails of his poorer subjects. “If God gives me life, I will ensure there is no labourer in my kingdom who has not the means to have a chicken in his pot each Sunday!” he is said to have pledged.

In death, however, the much-loved monarch has caused disagreement and division. On Friday, the rifts that have for decades split historians, scientists, researchers and descendants of France’s pre-revolutionary ruling families – the Orléans and the Bourbons – were prised open again by a new book.

In Henri IV: The Mystery of a Headless King, authors Stéphane Gabet and Philippe Charlier claim to have solved the enduring enigma of what happened to the king’s remains – specifically, his head. They insist that a mummified head found five years ago in a box in the attic of a retired tax collector, Jacques Bellanger, is that of Henri.

Facials hairs, a large beauty spot, a broken nose, a knife gash to the upper lip from an assassination attempt, all point to the skull being his. “Rubbish,” cry critics, who insist that the book owes more to fiction than fact and point to a lack of scientific proof and the fact that the brain – albeit shrunken to the size of a walnut – was still present, when it would have been removed by royal embalmers.

Meanwhile, the head of the man who may or may not have been king – and who may or may not have said on converting back to Catholicism for his coronation that “Paris is worth a mass” – sits in a bank vault near the Bastille where, symbolically, the mystery is rooted.

After his death on 14 May 1610, Henri IV was buried with previous kings in the Saint Denis basilica outside Paris. In 1793, French revolutionaries dug up his remains and tossed them unceremoniously into a mass grave.

It is unclear exactly when Henri’s head was separated from the rest of his corpse, but when the public grave was opened in 1817, it was missing. A head said to be his came to light in 1919, when Joseph Emile Bourdais, a photographer, bought it at auction for three francs.

Bourdais spent much of his life unsuccessfully trying to confirm its authenticity and offered it to the Louvre before his death in 1946, an offer that was refused. Then in 2008 a head was found in the attic of a house in Angers, western France belonging to Bellanger, who claimed to have bought it in 1953.

“In the loft, in an old wardrobe, was a box,” Gabet says. “Inside, there was something wrapped in an old towel. Jacques Bellanger folded back one side of the towel, then the other. The mummified head appeared, well conserved, impressive. It was a magic moment.”

The author says experts later reported: “There were a few long moustache hairs still visible and others broken at the roots. The nose was broken and bent to the left … the head was a brownish colour, with the mouth wide open … Almost all the soft tissue remained.”

Although the head had not been cut to remove the brain or embalmed – as would have been expected – a facial reconstruction from the skull matched portraits of the king.

Charlier, a pathologist who is an expert on historical remains, said that DNA tests on samples from the head and a trace of blood from Louis XVI linked the two royals.

“The DNA is clear: the mummified head found again in 2008 has the same genetic heritage as the dried blood of Louis XVI found on a handkerchief the day of his decapitation in 1793,” Charlier said in a statement.

There are now calls for the head to be reburied in the Saint Denis basilica, but while the scientists continue to argue, it remains in a Paris bank vault.

The row has split descendants from France’s two royal dynasties. Henri d’Orléans, Count of Paris and Duke of France, described the book as a “pseudo inquiry”. “This affair seems closer to a novel than scientific or historic truth,” he told French journalists. “What are we supposed to see from this supposed facial reconstitution – that he had a Bourbon nose?”

Prince Louis de Bourbon, Duke of Anjou, disagrees. “Now that we have found and authenticated the head, we have to organise its return [to the rest of the body],” he said.

However, Dr Olivier Pascal, president of the French Institute of Genetic Testing, told the newspaper Le Figaro that there was still no conclusive proof that the head had belonged to Good King Henri. “This information wouldn’t stand up in a court of law,” he added.