A dishonest policeman found 10 historic gold coins while metal detecting and cheated a landowner by selling them for £15,000, a court heard.
PC David Cockle, who currently lives in Leigh, had a contract with the landowner which allowed him to search his fields in return for splitting the proceeds of anything he unearthed.
But the 50-year-old amateur treasure hunter decided not to honour the deal after finding the coins dating back to the sixth or seventh centuries.
Ipswich Crown Court heard he did not tell the landowner about how he dug up the ancient Merovingian Tremissis coins from Gaul which is now France.
Instead the Norfolk officer secretly sold the coins to a dealer for £15,000 and kept the entire amount.
Cockle, of Pennington Wharf, also failed to report his find to the coroner who would have had to consider if the haul was counted as a treasure trove.
The coins which he sold in three batches over 14 months are believed to have been part of a larger hoard.
Another 34 similar gold coins were found in the same field in west Norfolk by another metal detecting fan who also had permission to be on the land.
But unlike Cockle, the other man did the right thing and reported his find to the authorities, leading it to be declared as treasure trove.
Sources said that the two finds taken together potentially made it the largest ever hoard of the type of coins ever found in the UK.
Cockle had initially denied stealing 10 coins belonging to another person, but changed his plea to guilty on the day his trial was due to start.
Judge Rupert Overbury adjourned sentencing until March 8 for a pre-sentence report, but told the officer that he was considering passing a custodial sentence.
The judge added that there was a rigid process to be followed if treasure was found.
But he said Cockle had sold the coins in batches to the dealer on the basis that he legitimately owned them.
Cockle also denied three charges of converting criminal property. But prosecutors said that they would not proceed with those charges. He was remanded on bail pending sentence.
Nicholas Bonehill, defending, said Cockle had been a serving officer, but that the situation was likely to have changed by the time he was sentenced.
A Norfolk Police spokeswoman said Cockle was suspended from duty and faced misconduct proceedings. She added: “The investigation was launched after the Norfolk and Suffolk Anti-Corruption Unit received information from a member of the public and Cockle was arrested in November 2015.”