Two metal detectorists who unearthed an astonishing hoard of gold jewellery, silver ingots and coins buried more than 1,000 years ago by a Viking warrior in Herefordshire face prison after being found guilty of theft.

George Powell and Layton Davies should legally have declared the find, estimated to be worth more than £3m, but instead they began to show it to dealers and tried to sell parts of it off.

Among the jewellery, which dated from the fifth to ninth centuries, was a ring, an arm bracelet and a small crystal ball held by strips of gold that would have been worn as a pendant.

The jewellery and one ingot have been recovered but the vast majority of the 300 Anglo-Saxon coins that police believe were found remain unaccounted for, to the frustration and anger of historians who see the hoard as hugely important.

Powell and Davies made the discovery on land owned by Lord Cawley near Leominster in June 2015.

The Guardian broke the news of the verdicts to Lord Cawley, who could be in for a reward as the treasure was found on his property. He said: “I suppose it’s good news for me. I could possibly be in for a reward. It would be nice.”

Cawley said he had not given the pair permission to detect on his land. “What they did was illegal, they shouldn’t have done it.” He doubted the missing coins would be recovered. “I don’t suppose they’ll find them now.”

Experts say the pair would have been handsomely rewarded had they handed the coins in.

Instead they are due to be sentenced on Friday along with two others who were involved in hiding the hoard. They could be given prison sentences.

Worcester crown court heard that after making their find of a lifetime, Powell and Davies did not speak to the authorities about their discovery but set about selling the coins.

Emperor coins valued at £100,000. Photograph: Supplied

They showed them to a coin dealer in Cardiff, Paul Wells. Police later found five of the coins at his home.

Simon Wicks, an antiques dealer and metal detectorist from East Sussex, was asked to help them sell the coins, the prosecution said. They included two extremely rare “Two Emperors” depicting King Alfred the Great of Wessex and Ceolwulf II of Mercia.

Wicks took them to a dealership in Mayfair, central London. An expert gasped when he saw the coins and valued each of the Two Emperors at £100,000.

Rumours began to circulate about the find. The gold and one ingot was handed over but police have recovered only 30 of the 300 coins.

Gareth Williams, a specialist at the British Museum on the Anglo-Saxons and the Vikings, said it was a great shame that the detectorists had not declared the find and that a large part of it was still missing.

“It’s not just a theft of the items,” he said. “If we don’t recover everything it’s a theft of our history. The stupidity is that our treasure system is the most generous in the world in terms of providing rewards for those who abide by the law.

“These men would be rich by now if they had done things by the book. They have chosen not to and in doing so have destroyed an important part of our history. It’s difficult to feel any sympathy for them at all; they have been greedy and selfish and the nation is the loser.”

Immediately following the verdicts, Wells was allowed to leave the dock after becoming unwell, and an ambulance was called for him by court staff.

The other three men were remanded in custody with Judge Nicolas Cartwright expressing concerns that if they were released on bail before being sentenced they could try to hide the missing coins, which are believed to be worth millions of pounds.

Lesley Milner of the Crown Prosecution Service said: “The coins Davies and Powell found were more than 1,100 years old. This find had immense historical value and should have been disclosed to the relevant authorities. But Davies and Powell actively hid their haul for their own selfish gain.”

Mike Heyworth, director of the Council for British Archaeology, called for stiff sentences for so-called “nighthawkers”. “All these finds add to our knowledge of the past. They also have particular significance to the local community – it’s their history and they are keen to have access to it it in their local museum.”

Powell, 38, of Newport, south Wales, and Layton Davies, 51, of Pontypridd, south Wales, were found guilty of stealing the jewellery, ingots and coins. Along with Wicks, 57, of Hailsham, East Sussex, they were convicted of conspiracy to convert criminal property. The three, plus Wells, 60, of Cardiff, were found guilty of conspiracy to conceal criminal property.