A scrap of twisted silver found a few weeks ago by a metal detector in Lancashire will take its place among masterpieces of medieval art at the British Museum, in an exhibition opening this week of the bejewelled shrines made to hold the relics of saints and martyrs.

The badge made of silver found by Paul King, a retired logistics expert, is a humble object to earn a place in an exhibition called Treasures of Heaven, but it is unique. It will sit among gold and silver reliquaries studded with gems the size of thumbnails – or the sockets from which they were wrenched by thieves – once owned by emperors, popes and princes.

The badge, the only one of its kind ever found in Britain, provides a link 500 years ago between this corner of rural Lancashire and the great pilgrimage sites of mainland Europe. It shows one of the companions of St Ursula, one of the most popular mystical legends of medieval Europe. She was said to be a British princess who sailed with 11,000 virgin companions to marry a pagan prince in Brittany, but diverted to go on a pilgrimage to Rome – and in some versions of the story, Jerusalem.

After many adventures they came to Cologne, where all were slaughtered by Hun tribesmen. When a large cemetery of Roman era bones was found in the city in the 11th century, they were declared the remains of the saint and her companions, and her cult spread across Europe.

King, a member of the South Ribble metal detecting club, found the silver plaque at the end of April in a field some miles from his home in Walton-le-Dale, where he had already found several hundred Victorian coins, but returned with the blessing of the landowner for a sweep with his new more high-powered metal detector.

"I knew immediately she was something special," he said. "I think she was hidden deliberately – she was folded over, not damaged by a plough strike in any way. It is extraordinary and moving to think how much history is locked up in this little piece of metal."

Although a church in Cologne holds her shrine and a whole chapel still decorated with the supposed bones of her companions, there were so many bones that the relics spread across Europe and beyond. Some of the most beautiful reliquaries, life sized busts of fashionably dressed young women, were made to hold the bones. The badge from Lancashire is a representation of just such a shrine - and so close in style and early 16th century date that it may come from the same Bruges workshop as the one in the exhibition on loan from the Metropolitan Museum in New York.

The Metropolitan reliquary, of a gently smiling young woman with her hair in a modish plaited style, is so alluring it has become the exhibition poster. The badge would have been bought as a souvenir by the Lancashire pilgrim from just such a shrine.

British Museum curator James Robinson said he was "beside myself with excitement" when he saw an image of the find. "To be honest if I hadn't been working on the exhibition it might have taken me a while to clock it – as it is I recognised her immediately as one of the companions of St Ursula. I hesitate to call it a miracle, but it is a most extraordinary coincidence that this should turn up just at this time."

He believes it is even possible that a similar reliquary may have been the centre of a shrine in Britain, destroyed as the cult of relics was condemned as idolatrous and blasphemous by religious reformers.

"The badge may be the only fragile, ephemeral piece of evidence for a cult of St Ursula in the north of England, that might have had at its centre a bust reliquary of continental manufacture."

The exhibition will include reliquaries which the faithful believed once held the breast milk of the Virgin Mary, the umbilical cord of the baby Jesus, the arm of Saint Luke - holding a golden pen to symbolise the gospels he wrote - and many still containing fragments of wood claimed to come from the cross on which Christ died. A carved icon of the Virgin which according to tradition was taken from the neck of the dead emperor Charlemagne, was one of the treasures of Aachen cathedral until it was given as present to Napoleon's Josephine. Some of the loans have never before left the churches or villages where they have been venerated for centuries. Many were believed to have miraculous powers, and made the places that held them wealthy pilgrimage sites - as Canterbury cathedral was for the relics of the martyred Thomas a Becket, and Santiago de Compostela in Spain remains to this day.

King, who has always been interested in history and spends days researching his finds in museums and archives, reported it under the Portable Antiquities scheme which encourages metal detectors to report all their archaeological finds, but she proved to be silver and so legally treasure which must be reported. When valued - the price will be shared between King and the landowner - Robinson hopes the British Museum will acquire her to find a permanent resting place in its medieval galleries.

Treasures of Heaven, British Museum, London, 23 June – 23 October