The former Archbishop of Canterbury used to hang a large copy of it around his neck and the Somerset town where it was unearthed named a theatre and a street in honour of its "treasure".

But it emerged yesterday that the silver amulet which the people of Shepton Mallet thought was a priceless early Christian artefact is a fake, possibly planted by a hoaxer who was trying to stop a warehouse being built on the site of a Roman cemetery.

The amulet was found 18 years ago and heralded as one of the most important Christian artefacts uncovered in the UK. But Liverpool University researchers have established that the silver was not of Roman origin and may have been produced in the 19th century or even later.

County council officials were trying to put a brave face on the findings but the disappointment, not to mention a little embarrassment, is palpable. Stephen Minnitt, the acting head of museums, said: "Experts are 99% certain that the amulet is not genuine and it is possible we may never be able to say with certainty how it came to be buried in Shepton Mallet."

He said the hoaxer was clearly something of an expert and may have been inspired by an object in the British Museum called the Sussex brooch, with which it shares some design features.

At the time of the cemetery dig, controversial plans were afoot to build a drinks warehouse on the site. Minnitt said the hoaxer may have hoped the discovery of a precious artefact could halt the development. It didn't work - the warehouse was built.

Jeanette Marsh, the deputy leader of Shepton Mallet town council, admitted: "It's like the magic has been removed from Shepton Mallet. It's a shame the myth of the amulet has now burst. It was part of the town's claim to fame but we're still proud of Shepton."

The amulet was unearthed in a small late-Roman cemetery in the grave of a man aged between 30 and 50 during excavations in 1990. Designed to be worn as a pendant, the front of the amulet was marked with the "ChiRho" - an early Christian symbol incorporating the first two letters of Christ's name in Greek.

It was considered important enough for George Carey, then the Bishop of Bath and Wells, to wear a copy which was double the size of the original. He continued to wear it when he became archbishop.

Doubts surfaced in the late 1990s after the British Museum carried out tests on the metal composition of the artefact, but it could not prove it was a fake. For the moment, the amulet is stored at the county museum in Taunton, which is closed for refurbishment. It will go back on display when the museum reopens in 2010.