The British Museum had a number of its artefacts stolen and put up for sale on eBay, with some items selling for as low as £40.
Museum officials launched an investigation after some of its items were posted on the online marketplace in 2016.
An antiques expert told the institution that they suspected a member of staff was stealing from the museum, before a staff member was later fired.
The dismissed employee has been named as Peter Higgs, 56, who was the museum’s curator of Greek collections.
Peter Higgs had been working at the museum for over 30 years.
The museum believes him to be guilty of stealing tens of millions of pounds worth of items for the museum.
Higgs had been working at the museum for over 30 years.
The institution said the items included “gold jewellery and gems of semi-precious stones and glass dating from the 15th century BC to the 19th century AD”.
One piece of jewellery dating back to the Roman age was put up on the website with a minimum price of £40 in 2016, but reportedly received no bids.
A dealer estimated that the item, made from onyx, a semi-precious stone, could be worth up to £50,000.
The museum has said it will be taking legal action against the former employee.
Greg Higgs, 21, the dismissed employee’s son, said his father maintained his innocence.
“He’s not done anything,” the son said. “He’s not happy about it at all. He’s lost his job and his reputation and I don’t think it was fair. It couldn’t have been [him]. I don’t think there is even anything missing as far as I’m aware.
Hartwig Fischer, director of the British Museum, stressed their commitment to getting the stolen pieces back. She said they are ready to “throw our efforts into the recovery of objects”.
She called the incident “highly unusual” and said the museum is “working alongside outside experts to complete a definitive account of what is missing, damaged and stolen".
Chair of the museum, George Osborne, said the trustees were “extremely concerned when we learned earlier this year that items of the collection had been stolen”.
One of the items uploaded to eBay was made of Onyx and is believed to be worth up to £50,000
"Our priority is now threefold: first, to recover the stolen items; second, to find out what, if anything, could have been done to stop this; and third, to do whatever it takes, with investment in security and collection records, to make sure this doesn't happen again," he added.
The majority of items stolen were small in size, and were kept in storerooms and not out on public display.
Professor Martin Henig, an expert on Roman art from the University of Oxford, said that many of the museum’s smaller items were not part of its online inventory.
He urged the museum to make a proper catalogue to ensure this doesn’t happen again.
Christopher Marinello, an expert in recovering stolen art, also doubted the museum’s security procedures.
He said: "It's not enough to have cameras on the walls, one needs to properly vet their employees. There needs to be sign-ins and sign-outs for every object that's being studied.
"There are plenty of things that museums need to do, and if any museum knows how to do it, it's the British Museum," he concluded.