It is Jesus's "Holy Towel", once visited by pilgrims in the belief that it showed the face of Christ, formed when he dried his wet head on a piece of cloth and left an indelible mark.
The Christian relic the Mandylion of Edessa usually takes pride of place in the Pope's private Matilda chapel in the Vatican. It is rarely seen in public, and is one of the earliest images of Jesus – although there is scholarly disagreement about whether the facecloth is the original or a copy made 400 years after the life of Christ.
Now, the British Museum has acquired permission to exhibit the Mandylion in Britain for the first time. In June, the cloth will be the centrepiece of the museum's Treasures of Heaven exhibition, showcasing assorted Christian relics. The museum is planning for vast crowds of Roman Catholics, Russian Orthodox and Coptic Christians, in keeping with the levels of excitement that such relics can attract.
"This is one of the most extraordinary loans in recent memory," said the director of the British Museum, Neil MacGregor. "The exhibition is all about trying to represent the universal human desire to reach out and touch the absolute."
Curator James Robinson compared the reverence in which the cloth is held to modern-day idolatry.
"In the Middle Ages it would have been greeted in the same way that David Beckham's sweaty shirt would be greeted today," said Mr Robinson. "It is an ordinary object made extraordinary by the person claimed to have come into contact with it."
The object was the subject of pilgrimages in the Middle Ages, say British Museum experts.
The cloth is among the most significant relics to be exhibited in Britain. In 2009 the remains of Roman Catholic nun St Therese of Lisieux arrived in England for a month-long tour. The nun's remains had been preserved since her death in 1897, and arrived in its original casket. The relic attracted hundreds of thousands of pilgrims to cities including Plymouth, Birmingham and Cardiff.
Such objects can have bizarre and often uproven links to Christ. Reims Cathedral once claimed to be in possession of the direct imprint of Christ's buttocks on a stone, and offered visitors the chance to kiss it.
The Mandylion of Edessa is believed to have been created after King Agbar of Edessa, now the Turkish city Urfa, asked an unknown painter to go to the Holy Land to paint Jesus. According to legend, the painter was unable to capture Christ's image because he was so dazzled by the light shining from his face. Instead, Christ wiped his face on a towel after washing himself and left an image behind. When the cloth was returned to Agbar, it is believed to have cured him of leprosy.
Some believe the Vatican object is the original; others claim it is a copy created in the fifth century. It is thought to have once been on display at Constantinople's Imperial Palace and transferred to the Vatican in the 14th century.