A former British Museum staff member accused of stealing artefacts at the attraction is not co-operating with recovery efforts, according to the museum's chairman.
An independent review on Tuesday recommended the museum document all of its artefacts after it was revealed some 1,500 items from the collection are missing, stolen or damaged.
A former employee at the museum is accused of stealing some artefacts. He strongly denies wrongdoing, with relatives saying he is innocent and “devastated” to lose his job.
George Osborne, museum chairman, told the BBC: “One of the things that we've got to get to the bottom of is exactly the motivation of the individual who we believe is responsible.
“But he has not been talking or co-operating."
A police investigation is underway, while the museum is also taking legal action.
The review, led by Sir Nigel Boardman, Chief Constable Lucy D'Orsi and Deputy High Court Judge Ian Karet, said that of the 2,000 items, some 1,500 are missing or stolen.
Around 350 items have had portions removed, such as gold mounts for gems.
Museum officials believe these portions are likely to be unrecoverable because they have probably been sold for scrap, the report adds. Around 140 items have been damaged by tool marks.
Of the 1,500 missing or stolen items, 351 items have already been returned and more than 300 further items have been identified.
A key target of the thefts, which took place over a long period of time, were unregistered items - mainly gems and jewellery - in the Department of Greece and Rome.
The museum's interim director Sir Mark Jones has previously disclosed that one million artefacts are unregistered.
The museum was alerted to suspicions of thefts in 2021 by academic and antiquities dealer Dr Ittai Gradel but an initial investigation incorrectly concluded that there was no basis to the claims.
Later that year, a spot check by internal audit revealed an item not in its proper location within the Greece and Rome strongroom, leading to a wider audit of the strongroom as well as the Greece and Rome jewellery and gem collection.
The audit, which began in April 2022, subsequently revealed further evidence of missing objects.
The report cannot be published in full because of the police investigation but the review recommendations said: “The museum should identify the unregistered or inadequately registered objects within the collection and register them fully.”
More than a third of the published recommendations are already under way or completed, the British Museum said.
Mr Osborne, the former Chancellor, said: “This review shows the British Museum is putting our own house in order, indeed we commissioned it because we were determined to learn the lessons of what went wrong.
“The British Museum was the victim of thefts over a long period, and we apologise again that this was allowed to happen.
“The ongoing police investigation means the full report cannot be published today, but we have accepted the recommendations in full, and have started to recover hundreds of the stolen items."
