Of all the world's great writers, Petrarch is the best known for losing his head. On Good Friday in 1327, the then 23-year-old writer and scholar fell madly - and forlornly - in love with a woman he saw in a church congregation.

His bad luck, to become enamoured of a woman who did not return his affections, was the rest of humanity's good fortune. For, in seeking to express his feelings for the woman he called Laura, Francesco Petrarch gave definitive form to the sonnet and established himself as the first modern, western poet.

Now, it seems, he has lost his head for a second time.

Scientists who have been examining what they thought were Petrarch's remains have discovered that the skull belongs to someone else. And they suspect it could be that of a woman.

Professor Vito Terribile Wiel Marin of Padua University, who is heading the investigation, told the Guardian yesterday: "This must have been robbery. It is not, frankly, a nice business."

The suspects in a literary whodunnit spanning almost 700 years include a bibulously larcenous 17th century friar and a supposedly clumsy 19th century anatomist. Death has put both beyond the reach of indictment, but if Petrarch's skull were to be traced as a result of the latest discovery it could lead to charges of receiving stolen goods, an offence for which, under Italian law, there is no statute of limitations.

The seeds of the mystery were sown last November when a crane lifted the lid from Petrarch's pink marble tomb at Arquà Petrarca, the town where he died in 1374. It was the latest in a series of exhumations in Italy of famous historical figures. Prof Terribile Wiel Marin helped to set the vogue when he examined the remains of St Anthony of Padua in 1981.

One of the main reasons for picking over Petrarch's remains was to reconstruct his face and create a definitive portrait in time for the 700th anniversary of the poet's birth on July 20.

"Since we now don't even have his skull, that is absolutely impossible," Prof Terribile Wiel Marin lamented.

The bones of what was thought to be Petrarch's venerable head were in fragments when they were removed from his tomb. In 1873, it had been opened by an investigator, Professor Giovanni Canestrini, also at Padua University.

"He claimed Petrarch's skull disintegrated on contact with the air," said Prof Terribile Wiel Marin. "Since none of us has ever come across an instance of this happening, we can only conclude he dropped it."

Or might he have made up the whole story, putting back a damaged substitute and keeping for himself the head of a man revered as one of the fathers of the Renaissance?

It was when the skull's fragments found in Petrarch's tomb were reassembled that doubts surfaced about their true nature. Prof Terribile Wiel Marin said one of his team, Dr Maria Antonia Capitanio, noted the contours in two areas - above the eyes and below the ears - were more typical of a woman. Samples from a tooth and a rib were sent for analysis by Dr David Caramelli, a molecular anthropologist at Florence University who compared fragments of their DNA.

Last Friday, he reported back his sensational findings. "I am sure that the two samples are from different people," Dr Caramelli said yesterday.

But could the tooth belong to Petrarch and the rib to someone else? Not a chance, said Prof Terribile Wiel Marin, the rest of the reassembled skeleton bore evidence of injuries mentioned by Petrarch during his lifetime, including one received from the kick of a horse on his way to Rome in 1350.

"There's no doubt about the body," Dr Caramelli said.

Although Prof Canestrini and his 19th century assistants are clearly in the frame, this is a crime with many possible suspects. Petrarch has been quite as much troubled in death as he was in life.

Before his body reached his tomb it spent six years interred in the cathedral at Arquà. In 1630, a drunken friar called Tommaso Martinelli, helped by four accomplices, broke in through a corner of the tomb and took some bones, apparently for resale.

Martinelli and his confederates were arrested, tried and exiled. But the missing remains were never recovered.

Could they have included the skull? Perhaps. But would someone cunning enough to have returned a phoney skull not have put back the other missing bones too?

At least two other small-scale robberies appear on the records. And, as Prof Terribile Wiel Marin noted, those are only the ones that are known about.

"Arquà is a pretty quiet place," he said.

Extracting a skull would require either the creation of a sizeable hole in the side of the tomb or the lifting of its two-tonne lid, both operations that would generate a suspicious amount of noise.

To narrow the field of suspects, Prof Terribile Wiel Marin said he had sent samples of the skull for radio carbon dating at the University of Arizona.

"If we find out that the skull dates from, say, 1720, then we know to exclude everyone who died before that," he said.

From this investigation, it would seem, no one should expect swift results.