The total value of artefacts now known to be missing runs into millions of pounds, it was understood.
Staff are shocked at the scale of the losses, which were unearthed during an internal investigation into a senior curator at the museum who was dismissed in July.
The museum has so far refused to make public the number of items that were stolen, or to release photographs or descriptions of them. However, sources have confirmed to The Telegraph that the true number is well over 1,000 and “closer to 2,000”.
The sheer number of missing objects, some of which are 3,500 years old, helps to explain why the museum was so reluctant to share all of its information with the public.
As well as being deeply embarrassing for the museum, it appears increasingly likely that it might never know exactly what has been stolen because of gaps in its inventory.
The latest revelation will further intensify calls for Hartwig Fischer, the British Museum director, to resign immediately rather than wait until 2024. He announced in July that he was going to “pass on the leadership” next year.
Staff also believe that the position of Jonathan Williams, the deputy director, is now untenable. He was personally warned about the thefts in February 2021.
Martin Henig, a leading expert on Roman art at the University of Oxford, said the scale of the losses was “horrifying”.
He said: “This is the worst case that I’ve come across like this because it involves not just selling the odd object, but also destruction. This is totally unforgivable.”
He added that the loss of so many objects was a huge blow to historians because “although they’re small, they were particularly valued in antiquity and they tell us more than sculpture about life in the past. They were very intimate items”.
One museum source said the evidence is worse for the directorate than had been realised until now, and that its handling of the case can only be described as “negligent and incompetent”.
Challenging the British Museum’s official line that it had taken immediate action when the thefts were discovered, they claimed that evidence presented to them was actually ignored.
As The Telegraph reported last week, an antiquities expert had told the museum three years ago that items from its collection were being offered for sale on eBay.
One Roman object, valued at £25,000 to £50,000 by dealers, was offered for just £40.
One source said: “The directorate has been banned from the investigation and has nothing to do with it… That the museum’s own management is banned from having any involvement with this investigation is extraordinary. That goes to show that the trustees have apparently lost all trust in the directorate.”
Until now, the museum has only said that an unspecified number of objects dating from 1,500BC to the 19th century, and comprising jewellery made of gold, semi-precious stones and glass, had been stolen or damaged.
It also disclosed that a member of staff – since identified as Peter Higgs, its curator of Mediterranean cultures – had been sacked. Higgs, who had worked at the British Museum for more than 30 years, has denied any wrongdoing, according to his family.
The thefts are being investigated by police. No arrests have been made.
An independent review led by Sir Nigel Boardman, a former trustee, and Chief Constable Lucy D’Orsi, of the British Transport Police, will “provide recommendations regarding future security” and “kickstart – and support – a vigorous programme to recover the missing items”.
A British Museum spokesman said: “This is all now subject to a police investigation as well as the independent review. We cannot comment further.”