Watton Viking discovery by Jason Jones from Norwich, Norfolk

Jason Jones, 44, from Norwich, made the find while searching a field near Watton in January this year, having previously found two medieval silver coins there.

The construction industry worker, who was with his wife Lisa, said he had forgotten to charge his main detector and was using his backup machine.

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The die, which was used to make decorative patterns on helmets during the Viking age (Image: PA)

"I returned to the area where the coins were found and got a loud signal, and at a depth of just two inches found an unusual bronze object," he said.

"Lisa came over and was speechless when she saw it.

"Neither of us had any idea what it could be, but that evening after posting a picture on Facebook we realised it was Viking in date and notified the local archaeologist to have it recorded".

Jason with his two daughters Ela and Lia, who often go metal detecting with him (Image: PA)

The bronze artefact, called a die, would have been used to create decorative motifs to be applied to a military helmet, according to Nigel Mills, a specialist at Noonan's auctioneers, where it is to be sold.

"The die is used to replicate the design into a thin sheet of metal which is pressed into the die and then has the intricate design formed," he said.

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"You can see this decoration on important helmets like the Sutton Hoo helmet".

The die is to be offered for sale at Noonans Mayfair in London on July 18, with a guide price of £16,000 to £24,000.

Mr Jones said he bought a metal detector for his son Rio’s 15th birthday four years ago, and within a few weeks the whole family, including his two daughters Ela and Lia, were detecting every weekend.