Craig Best, 46, and Roger Pilling, 75, were convicted of conspiring to sell criminal property, namely ninth century coins believed to have been buried by a Viking.

The items, worth £766,000 and which are of "immense historical significance", have never been declared as Treasure and have not been handed to the Crown.

Jurors at Durham Crown Court also convicted both men of separate charges of possessing criminal property, thought to be part of a larger trove of 300 coins worth millions of pounds known as the Herefordshire Hoard.

Just one third of the collection, unearthed in Leominster, has been recovered.

Best, of Bishop Auckland, was arrested with three coins at a hotel in Durham in May 2019, as part of a police sting.

He believed he was meeting a metals expert, hired by a broker working for a wealthy US buyer - but in fact, he was speaking to an undercover detective.

Pilling, who ran an engineering company, was arrested at his home in Loveclough, Lancashire, where a further 41 coins were seized.

These 44 coins are thought to have originated from the Herefordshire Hoard, a find worth millions of pounds, which was discovered in 2015.

Four others have already been convicted for their roles in concealing the find - including George Powell, 41, and Layton Davies, 54, who were jailed for more than 18 years at Worcester Crown Court in November 2019.

The police launched an undercover operation after Best tried to sell coins to a genuine American collector, who contacted UK-based experts about the availability of extremely rare and valuable examples - before the authorities were notified.

Best and Pilling were charged in August 2021 and stood trial this month.

Judge James Adkin adjourned the case, telling the defendants that the sentencing would be "complicated" due to the rarity of the offences.

He remanded both men in custody, saying: "You have both been convicted of what I consider to be compelling acts of serious criminality, in relation to these artefacts.

"You are both aware of what the sentence is likely to be, imprisonment for years."

Coins will 'transform' our understanding of history

The coins are thought to have been made between 874 and 879 CE and include two extremely rare examples of two-headed coins, depicting King Alfred of Wessex, known as Alfred the Great, and the last Mercian king, Ceolwulf II - a figure discredited by Saxon writers as a Viking puppet ruler.

Experts believe the coins indicate Alfred must have had an alliance with Ceolwulf - before he was largely erased from the history books by Alfred's court.

The Mercian ruler disappeared from history in 879 CE, with Alfred inexplicably recorded as ruling a large part of Ceolwulf's kingdom.

The find casts doubt on the popular belief that Alfred the Great saved England almost single-handedly from the Vikings.

Read more:
New coins featuring King Charles III portrait revealed
Huge Anglo-Saxon burial site discovered under HS2 construction

Dr Gareth Williams, curator of the Early Medieval Coins and Viking Collections at the British Museum, said the coins were "very much part of our heritage" and had "transformed our knowledge and understanding" of late ninth century politics.

"The coins show beyond any possible doubt that there was a political and economic alliance between Alfred and Ceolwulf II.

"Together, the two kings carried out a major reform of the coinage, introducing high-quality silver coins, with the Two Emperors design symbolising this alliance, followed by a second joint coinage.

"As more coins emerge, it is clear this monetary alliance lasted for some years," Dr Williams added.

"The theft of finds like this are not just a theft from the landowner, who [has] rights, it is a theft of our heritage.

Following the conviction, Durham Constabulary's Detective Superintendent Lee Gosling said the investigation had been "lengthy and complex", adding: "It is astonishing the history books need re-writing because of this find.

"The coins come from a hoard of immense historical significance relating to the Vikings and we are delighted that they are now with the British Museum."