Two British men were jailed Friday for stealing a valuable hoard of 1,100-year-old coins and jewellery they found with metal detectors - much of which remains missing.

George Powell, 38, and Layton Davies, 51, failed to declare the find as required by law, and were sentenced to 10 years and eight-and-a-half years respectively.

The hoard discovered in 2015 on farmland in Herefordshire, near the English-Welsh border, are believed to be Anglo Saxon items buried by a Viking around 878 or 879.

Aside from their value, estimated at between £3 million and £12 million (3.5-14 million euros, $3.9-$15.4 million), they have helped reveal a new aspect of English history.

Five of the 31 coins recovered are exceptionally rare, depicting King Alfred the Great of Wessex and a lesser known monarch, Ceolwulf II of Mercia, sitting together.

They were previously not known to have had an alliance, according to Gareth Williams, curator of early medieval coins at the British Museum.