The British Museum staff member suspected of stealing and damaging thousands of items is not co-operating with the search to recover them, according to the chair of trustees George Osborne.

An independent review into the thefts has recommended the institution completes the documentation of its collection and closes any gaps in the registration of objects.

It was revealed in August that an estimated 2,000 items from the collection, worth millions of pounds, were found to be missing, stolen or damaged and that police were investigating.

An unnamed member of staff has been sacked and the museum is taking legal action.

Mr Osborne 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.”

He added: “We were the victim of an inside job. We’re determined to learn the lessons, make sure the collection is safe and be a more open, transparent and self-confident institution going forward.”

The review, led by Sir Nigel Boardman, Chief Constable of British Transport Police Lucy D’Orsi and Deputy High Court Judge Ian Karet, says 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 and announced steps to upload digitally or improve 2.4 million records.

The museum was alerted to suspicions of thefts in 2021 by academic and antiquities dealer Dr Ittai Gradel but an initial investigation incorrectly concluded 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.

Mr Osborne acknowledged the lapse over Dr Gradel’s warnings, telling the BBC: “It was not taken as seriously as it should have been. And that’s a big lesson for us to learn.”

The report cannot be published in full because of the police investigation but the review recommendations say: “The museum should identify the unregistered or inadequately registered objects within the collection and register them fully.

“The museum should introduce a policy regulating how registration should be undertaken – what information and in what detail artefacts should be registered.”

It added: “This policy should require newly acquired objects to be registered promptly.”

Implementation of more than a third of the published recommendations were already under way or completed, the British Museum said, including a plan to complete the documentation and digitisation of the entire collection within the next five years.

Mr Osborne 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.

“Above all, we’re determined to emerge from this period a stronger, more open, and more confident museum that is fit for the future. Thanks to the hard work of the review team, we’re now equipped to do just that.”

Sir Mark said: “This is a helpful set of recommendations, many of which we are already delivering on.

“No-one can pretend this has been an easy period for the museum, but I have the utmost admiration for the commitment of the staff to building a stronger future for the museum we all care so deeply about.”