The brooch looted by Viking more than a thousand years ago was embedded in a lump of organic material excavated from a site in Norway.
The Guardian newspaper says the ornate, gilded disc brooch dating from the eighth or ninth century was found by chance.
It has been described as a ‘staggering find,’ which was unknown as it lay in the storeroom.
The report says the brooch was concealed in a lump of organic material excavated from a Viking burial site at Lilleberge in Norway by a British archaeologist in the 1880s. It was acquired by the British Museum in 1891.
The paper reports that Curator Barry Ager, a noted Vikings specialist, was poring over artifacts before a visit from a Norwegian researching the Viking site.
His eye was caught by some metal sticking out of the side of the organic lump and he asked the conservation department to X-ray it.
Ager told the paper: “At that stage, I really didn’t know what was inside. It was a staggering find.
“It turned out, quite remarkably, to be this Celtic disc. It’s extremely exciting. It’s a very rare example of its sort within the collection and shows contact between the British Isles and Norway in the Viking period, objects seized as loot in this country and taken back.”
Ager told the Guardian that he believes the brooch was originally made in Ireland or Scotland and came from a shrine or a reliquary.
He added that the Vikings converted it into a brooch by attaching rivet holes and a pin.
The brooch is almost 2.4 inches in diameter and had been buried in the grave of a high-status Viking woman.
The report says substantial remains of the gilding still survive on the top surface. Its elaborate design includes three dolphin-like creatures and interlaced patterns.
Ager revealed: “The patterns, the quatrefoil of the central roundel and the form of the ‘dolphins’ heads have clear parallels in Celtic metalwork and manuscripts of the 8th to early 9th centuries, such as the Tara Brooch and the Book of Mac Regol.
“The craftsmanship is very fine. The Vikings valued “eye-catching” objects: The Vikings themselves were very skilled metalworkers, so I’m sure that’s something that would appeal to a Viking eye.
“It was the custom to bury the person with their personal possessions. They were pagan at the time, so it was part of the standard Viking burial rite.”
The brooch will go on display at the British Museum in March.