A police raid on properties in County Durham and Lancashire has uncovered a Viking hoard, which has the potential to rewrite British history.

The £500,000 hoard of Viking-era coins and silver ingot was reclaimed as part of Operation Fantail, a wider investigation into illegal dealing in historical treasures. Under the terms of the Treasure Act 1996, finders of precious metal objects over 300 years old must report them to the local coroner as possible treasure. In recent years, many nationally important finds have not been declared.

Curator of early medieval coins and Viking collections at the British Museum, Dr Gareth Williams, said the coins from the hoard “add significantly to our understanding of the political history of England in the AD 870s” and of Alfred the Great, King of Wessex and his contemporary, Ceolwulf II of Mercia.

Alfred the Great was lauded for defeating the Vikings in AD 878 and unifying the Kingdom of England. Conversely, Ceolwulf has been viewed until now as a minor nobleman and a puppet of the Vikings.

The coins recovered in the recent raid tell a different story. They feature both Alfred and Ceowulf and are of the same design and type. The fact that both rulers used the same coin-issuing system suggests the men were equals in their fight against the Vikings and that Ceowulf was a proper king in his own right.

It was only after Ceowulf’s disappearance from historic records in 879 and Alfred’s takeover of his territory that Ceowulf developed an unflattering reputation in the annals of history. Chroniclers in Alfred’s court gave the Saxon King sole credit for defeating the Vikings and omitted to mention a possible alliance between Alfred and Ceowulf, which the coins suggest.

Detective Inspector Lee Gosling, senior investigating officer for Operation Fantail, said the recovery of the hoard represented the “very early stages of what is going to be a very long and complex investigation”. He added that Durham Constabulary were “delighted” to be able to hand over the treasures to the British Museum.

“This is an extremely unusual case and it is not every day we get the chance to shape British history”, Gosling said.