A cherished image of the Roman emperor Hadrian as a gentle, philosophical man wearing the robes of a Greek citizen has been shattered with one blow of a conservator's chisel at the British Museum.

The head, with its neatly trimmed beard and fringe of exquisitely crimped curls, is certainly Hadrian but it seems the body it has been attached to for almost 150 years belongs to somebody else. The statue, a unique piece that has been cited in many biographies of Hadrian as proof of his love for Greek culture and customs, and illustrated countless times, is an ingenious Victorian confection.

Thorsten Opper, curator of the exhibition on Hadrian which opens at the British Museum next month, said he initially felt "gutted" as Tracey Sweek, a stone conservator, delicately removed the layer of Victorian plaster masking the join of head and body.

As soon as the team saw that the carved draperies continued under the plaster, and how awkwardly the head sat into the neck socket, they knew immediately that the two pieces could never have belonged together.

Opper always intended the exhibition to re-examine the myths and truths about the Spanish-born general who became a highly unusual Roman emperor, but had not planned such a literal piece of iconoclasm.

The statue, which has been a star exhibit at the British Museum since the 1860s, was due for a check on its condition before the opening of the exhibition on the man who left Britain one of its world-famous monuments, the massive wall stretching coast to coast across northern England.

Hadrian, emperor from AD117-138, was a brutally pragmatic military leader, who put down a Jewish revolt with exemplary savagery, but was as interested in architecture as conquest and scattered spectacular building projects across Rome and the empire. He was married - to the niece of his predecessor, the empire-building Trajan - but openly gay, and declared his lover, Antinous, to be a god. When the beautiful young Greek man drowned mysteriously in Egypt, he founded a city in his memory.

The exhibition will include a ravishing portrait of Antinous on loan from the Vatican, and an inscribed stone from a temple to him excavated only in the past few years. Even in his day, there were suggestions Hadrian might have had Antinous murdered, because he had grown up enough for the relationship to be seen as shameful, or that, heartbroken and abandoned, Antinous had taken his own life.

Scores of portraits, all officially sanctioned images, show Hadrian either in military uniform, naked as the god Mars, or clad in a Roman toga. Several show him literally trampling the body of a barbarian under his feet.

However Hadrian was interested enough in Greek culture to earn the nickname "Graeculus", the little Greek, and the British Museum statue was held to be a unique illustration of that gentler, Hellenised Hadrian.

The statue was excavated by two British naval officers in 1861 from the ruins of a temple to Apollo in Cyrene, in what is now Libya. It was broken in several places, with the head of a goddess tucked between its feet. The Hadrian head was found nearby. All the fragments were shipped to London and reassembled.

"As a sculptor, once I looked at it properly the proportions were all wrong, the head was the wrong size for the body," Sweek said. She also believes the hands were added, possibly from two separate statues.

The British Museum workers were not consciously creating a fake, Opper believes. He thinks they were restoring the statue as they thought it had originally appeared, precisely matching the Victorian view of a weaker, less impressive figure than Trajan. In the exhibition the statue will be displayed with the head in place, and an explanation about Hadrian's borrowed robes.

"I felt awful for about 20 minutes because we were physically destroying a cherished vision of the kind of man Hadrian was, but we have to tell the truth as far as we understand it. That is what museums are for," Opper said. "Our exhibition will certainly show a much darker, grittier figure than the traditional character. But I don't think you ever find a nice, cuddly man that can rule an empire."

What the Romans did for us

Walls

Not just Hadrian's Wall, a tourist attraction drawing up to 750,000 visitors a year, but also the walls of London, Newcastle, York, and any town with chester in its name.

Roads

These routes often followed older straight tracks. They set new standards of engineering and maintenance and were intended to make tax gathering efficient and help speed up troop movements.

Plumbers

The word comes from the Latin for lead. The Romans brought with them not just hot baths - although the Celts were already keen on natural hot springs, such as those at Bath - but also flush lavatories, saunas and steam rooms.

Latin

A universal language for some time before txt spk took over the world, and still the language of Christianity, law, and urban district council mottoes.

Fine dining

Instead of just boiling up a boar in a bronze cauldron, the Romans cooked fish, game and delicacies such as dormice served in honey, adding fruit, herbs, spices and dressings including a rotted fish sauce that was imported in vast quantities in amphorae. Archaeologists who recreated the recipe do not recommend it.