The British Museum said on Wednesday a member of staff had been dismissed after items from its collection, including gold jewelry and gems, had been found to be missing, stolen, or damaged.
The museum, one of the most visited in the world, said it was taking legal action against the individual and had also launched a review of security. London's Metropolitan Police is also investigating, the museum said.
It said the majority of the items involved were small pieces kept in a storeroom and included gold jewelry, gems, and semi-precious stones and glass dating from the 15th century BC to the 19th century AD.
None of the items had recently been on public display and were kept primarily for academic and research purposes, it added.
"An unusual incident"
"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," said Hartwig Fischer, Director of the British Museum.
"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."
The museum's chair, former finance minister George Osborne, Chair of the British Museum, said the trustees were extremely concerned when they had learned of the theft "earlier this year."