Following reports of “missing, stolen, or damaged” treasures, the British Museum in London fired a staff member and called the police, who are now looking into the matter.
Gold, jewellery, and gems made of semi-precious stones were among the items taken from the museum, one of the top tourist destinations in the UK.
The majority of the items were kept in a storeroom, the museum said.
British Museum director Hartwig Fischer said the museum would “throw our efforts into the recovery of objects”.
He added: “This is a highly unusual incident. I know I speak for all colleagues when I say that we take the safeguarding of all the items in our care extremely seriously.
“We have already tightened our security arrangements and we are working alongside outside experts to complete a definitive account of what is missing, damaged and stolen.”
Legal action would be taken against the staff member who was fired, the museum added.
The Economic Crime Command of the Metropolitan Police is investigating but no arrests have been made.
The British Museum has also started an independent review of security.
None of the items, which dated from the 15th Century BC to the 19th Century AD, had recently been on display and were kept primarily for academic and research purposes, the museum said.
The PA news agency said it understood the items were taken before 2023 and over a “significant” period of time.
George Osborne, chair of the British Museum, said: “The trustees of the British Museum were extremely concerned when we learnt earlier this year that items of the collection had been stolen.”
He added: “We called in the police, imposed emergency measures to increase security, set up an independent review into what happened and lessons to learn, and used all the disciplinary powers available to us to deal with the individual we believe to be responsible.”
Mr Fischer added the organisation had “brought an end to this”, and was “determined to put things right”.
The museum’s independent review will be led by former trustee Sir Nigel Boardman and Chief Constable Lucy D’Orsi, of British Transport Police.
They will provide recommendations regarding future security arrangements and start “a vigorous programme to recover the missing items”, according to the museum.
Sir Nigel said: “It will be a painstaking job, involving internal and external experts, but this is an absolute priority, however long it takes, and we are grateful for the help we have already received.”
Its collection spans six continents and two million years of history, including the Parthenon sculptures, also known as the Elgin Marbles – the fate of which is the subject of much discussion.