The British Museum has announced items from its collection were found to be missing, stolen or damaged.
The London museum, which houses artefacts from around the world, has launched an independent review of security as it said a staff member at the institution has been dismissed.
The matter is also under investigation by the economic crime command of the Metropolitan Police.
Legal action will be taken by the museum against the unnamed staff member, the institution said.
Hartwig Fischer, director of the British Museum, said: “The museum apologises for what has happened, but we have now brought an end to this – and we are determined to put things right.
“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. This will allow us to throw our efforts into the recovery of objects.”
In July, the museum announced that Mr Fischer, a German art historian, would step down from his role next year.
The items include gold jewellery and gems of semi-precious stones, and glass dating from the 15th century BC to the 19th century AD.
The museum described them as small pieces, which were not recently on public display and were mainly used for research and academic work.
George Osborne, chairman 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.
“The trustees have taken decisive action to deal with the situation, working with the team at the museum.
“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.
“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.
“This incident only reinforces the case for the reimagination of the museum we have embarked upon. It’s a sad day for all who love our British Museum, but we’re determined to right the wrongs and use the experience to build a stronger museum.”
The review will be led by former trustee Sir Nigel Boardman, and Chief Constable of the British Transport Police, Lucy D’Orsi.
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: “The recovery programme will work to ensure the stolen items are returned to the museum.
“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.”
The PA news agency understands that the items were taken before 2023 and over a “significant” period of time.
A spokesperson for the Met Police said: “We have been working alongside the British Museum.
“There is currently an ongoing investigation – there is no arrest and inquiries continue. We will not be providing any further information at this time.”
Items have gone missing from the museum in previous years including a number of coins and medals in the 1970s and a 1993 break-in when Roman coins were taken.
In 2002, the museum reviewed security following a 2,500-year-old Greek statue being stolen by a member of the public.
The institution said at the time that the Greek Archaic Gallery had been open to the public but there was no permanent guard on duty when the 12cm-high marble head was taken.
Two years later, Chinese gems also went missing.
In 2017, it was revealed a £750,000 Cartier ring diamond from the heritage asset collection had been reported absent in 2011.