The British Museum was told three years ago that a member of staff was stealing items from its collection after priceless objects were offered for sale on ebay as long ago as 2016.
An antiquities expert became suspicious after seeing a piece of Roman jewellery on the online auction site, and traced the seller to a department of antiquities at the museum.
Despite tipping off senior staff at the museum in 2020, the expert was appalled that no action was taken until earlier this year, when the long-serving staff member in question was sacked.
Police are now investigating the theft of a number of objects dating between 1,500BC and the 19th century AD, but the revelation raises serious questions for the museum, which is now in a race to recover the items before they can be put beyond reach.
Sources in the art world have told The Telegraph that the thief began selling items on ebay that were unregistered by the museum, meaning it would have been almost impossible to prove that they were stolen from the world-famous institution.
Starting in 2016, or possibly even earlier, semi-precious gems and glass items were coming up for sale, mostly from the Towneley collection of Graeco-Roman artefacts donated by an 18th century landowner.
But the thief got “sloppy”, according to one source, and started selling items that had been catalogued by the museum and were easily checked against their online inventory.
They included a fragment of Roman jewellery showing the head of a man in profile, and a similar piece, known as a cameo, with the portrait of a young Roman engraved on a gem made of plasma.
The seller was using a pseudonym similar to his real name, but his Paypal account linked to an account under the real name of the British Museum staff member, whose Twitter username was almost identical to the Paypal reference. On Twitter, the thief used their real name.
The expert, who does not want to be named, contacted the ebay seller in 2016 and challenged him by suggesting he knew his real name and identity, but the seller claimed ebay had got his details wrong and denied working at the museum.
Then in June 2020, the expert contacted a middleman and asked him to pass on his findings to the museum.
In October that year, he wrote to a British academic to express his frustration that nothing was happening.
He suggested at the time that the museum “may not be interested in knowing” because the thefts would be “hugely embarrassing” for them.
A spokesman for the museum said: “We have conducted a thorough investigation, identified the person we believe to be responsible, and that person has been dismissed.
“We are also taking further robust action to ensure this can never happen again. The whole question of thefts at the museum is now subject to a criminal investigation, so we cannot comment further.”
On Wednesday, the museum announced that it had sacked a staff member after a series of thefts of jewellery made of gold, semi-precious stones and glass, which dated back years.
George Osborne, the former chancellor who is now chairman of the museum, has instigated an independent review that will try to recover the gems, discover what went wrong and increase security to prevent it happening again.
The review will look into whether the museum responded quickly enough when it became aware of the thefts, and whether police were contacted at the earliest possible opportunity.
The thefts are being investigated by the Metropolitan Police, which has not made any arrests in the case to date.
Hartwig Fischer, the museum’s director, has issued a public apology for the loss of the items.