A father and son with metal detectors have unearthed the most important haul of Viking treasure since the 19th century.

It was buried in AD927 in North Yorkshire and its size and quality are described by experts at the British Museum, which announced the find yesterday, as of local, national and international importance and “an exceptional find”.

David and Andrew Whelan, from Leeds, and the farmer who owns the field in which they made the discovery, could reap a reward of up to £1 million once the treasure is formally valued by an independent committee.

The two men were searching the field, near Harrogate, in January only because they had been unable to get permission at another location. Not expecting to find anything – they had searched the field before – they were astonished when one of their metal detectors emitted a loud beep.

Getting down on their hands and knees, they unearthed 617 silver coins and 65 other objects, including a gold arm-ring and silver bars that would have been used as currency.

The most impressive part of the treasure is a silver vessel decorated with animals and vine-scrolls, which the museum said was probably from a monastery in what is now France. It could have been looted by the Vikings or given to them in tribute.

Most of the objects had been carefully jammed into the vessel, leaving little space for the earth to enter. That in turn had been protected by some form of lead container. As a result, the hoard was extremely well preserved.

Andrew Whelan, 35, a surveyor, said: “We knew it was a hoard and that it was very important, but never in our wildest dreams did we expect it to be as important as it is.”

Jonathan Williams, the British Museum’s keeper of prehistory and Europe, said that the “remarkable” find shed new light on a period overshadowed by bloodshed and brutality. “This was a crucial moment in the country we now live in,” he said. The treasure may have been buried for safekeeping by a wealthy Viking during the period of unrest that followed the conquest of the Viking kingdom of Northumbria in AD927 by the Anglo-Saxon king, Athelstan. He could have been killed – or even forgotten exactly where he buried it.

The artefacts came from as far afield as Afghanistan in the East and Ireland in the West, as well as Russia, Scandinavia and continental Europe, reflecting just how far the Vikings travelled from modern-day Norway and Denmark. “This is the world in a vessel,” Dr Williams said. “England was just one of the places they came to.”

The coins relate to Islam and to the preChristian religion of the Vikings, as well as to Christianity. Barry Ager, Viking curator for the British Museum, said: “One coin is inscribed Rex Totius Britanniae, which means king of the whole of Britain. It is the first time a king had been claiming to be king of the whole of Britain.”

The hoard was yesterday declared treasure under the terms of the Treasure Act (1996) by Geoffrey Fell, the Coroner for North Yorkshire, who said: “Treasure cases are always interesting, but this is one of the most exciting cases that I have ever had to rule on.”

The next stage of the process is for the hoard to be valued for the Department for Culture, Media and Sport by the independent Treasure Valuation Committee. The British Museum said that it would like to buy the hoard.

Asked how they would spend the money, Andrew Whelan said: “We’ll invest it. Being fairly cautious Yorkshire people, we won’t go and buy a sports car or anything like that.”

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