Durham Constabulary officers seized a large number of coins and a silver ingot, with an estimated value of at least £500,000, from properties in County Durham and Lancashire earlier this month.
The recovered material comes from a Viking hoard and is believed to be of major historical significance.
It contains coins of Alfred the Great of Wessex and his less-well-known contemporary Ceolwulf II of Mercia.
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King Alfred inflicted a major defeat on the Vikings in AD 878, and experts from the British Museum believe the coins belong to an undeclared hoard consistent with the location of the Viking army at that time.
The hoard is so important because it fills a gap in the understanding of history at this time.
Until now accounts have suggested Ceolwulf of Mercia was a puppet of the Vikings and a minor nobleman rather than a proper king.
However, the coins tell a different story and show two rulers standing side by side as allies.
Detective Inspector Lee Gosling, senior investigating officer for Operation Fantail at Durham Constabulary, said: “We are in the very early stages of what is going to be a very long and complex investigation.
“We believe the material recovered comes from a hoard of immense historical significance relating to the Vikings, and we are delighted to have been able to hand it over to the British Museum.
“This is an extremely unusual case and it is not every day we get the chance to shape British history.
“It is the legal responsibility of the finder of any precious metal objects that are over 300 years old to report them to the local coroner as possible treasure under the terms of the Treasure Act 1996.”
Dr Gareth Williams, curator of early medieval coins and Viking collections at the British Museum, said: “This is a nationally-important hoard. The coins I have seen so far add significantly to our understanding of the political history of England in the AD 870s.
“This is the period in which Alfred the Great (AD 871-99) was fighting the Vikings, but which also led to the creation of a unified kingdom of England under Alfred and his successors.
"The hoard contains coins both of Alfred and of his contemporary Ceolwulf II, King of Mercia (AD 874-9).
“Ceolwulf is described in unflattering terms in surviving sources written at Alfred's court some years later, but around the time the hoard was buried, probably in AD 879, Ceolwulf mysteriously disappeared, and Alfred then took over Ceolwulf's kingdom as well as his own.
“The coins point to an alliance between the two which the later sources from Alfred's court 'forgot' to mention, while at the same time stressing Alfred's new alliance with his former enemy, the Viking leader Guthrum.
“This is just the latest in a series of spectacular hoards that are helping to transform our understanding of history.
“The Treasure Act 1996 provides a system for reporting such finds, as well as for rewarding those finders who abide by the law.
“At the same time, there are legal sanctions for those who fail to report, and I'm delighted that Durham Constabulary have been able to trace this material and recover it.”
The police investigation is ongoing and a number of people have been arrested on suspicion of dealing in culturally tainted objects.