With a doctorate in Roman religion and a university chair, Ittai Gradel, who has died of cancer aged 61, might have confined his achievements to a successful scholarly career. However, in 2008, bored with routine bureaucracy, he left his post at Reading University, and returned to his native Denmark to deal in antiquities.

His disillusionment with academia was reinforced when, a few years later, he discovered that large-scale thefts had been taking place from the British Museum’s collections. At first reluctant to believe the accumulating evidence, Gradel contacted the museum in 2021 only when it became impossible to deny – and was told nothing was missing.

Ill and increasingly impatient, he took his cause to the museum’s trustees, and at last the police were called. In 2023 the director and deputy director resigned, and a senior curator was dismissed. Investigation of the loss or damage of 2,000 pieces of ancient goldwork, gems and glassware continues.

On leaving Reading, Gradel had at first joined an antiquarian book dealership in Copenhagen. But he soon changed his focus to Graeco-Roman engraved gems, finding them both interesting – they evoked the private experiences of people in antiquity, he said, yet could still be worn as personal jewellery centuries later – and profitable.

With hindsight, he thought a purchase he made at the very start, trading freelance from home, consisted of pieces stolen from the British Museum. By 2010 there were signs that might have halted a less trusting mind. An English vendor, claiming to be an old man who had inherited gems from his grandfather, kept finding more to sell. When Gradel tried to meet him (having failed to prove his existence any other way), he received a message supposedly from the man’s son who told Gradel that he had just died.

In 2016 someone with the same name as the allegedly deceased vendor listed a Roman cameo on eBay that Gradel knew from a 1926 British Museum catalogue: the listing disappeared within hours, and when asked about it, the vendor said the gem had belonged to his sister and was not his to sell.

Only in 2020, however, did Gradel finally accept that something was up. He saw a piece for sale online that had featured in the same 1926 catalogue, and found that the buyer had acquired it from the same vendor who had first provoked Gradel’s suspicions 10 years before. Hackles raised, Gradel searched his records and found a PayPal receipt naming a Peter Higgs, similar to the name he had been given on earlier sales - Paul Higgs or Higgins. A fellow dealer told him that this was the name of a British Museum curator.

When tipped off, the museum saw no reason to investigate. A few months later, it promoted Higgs to acting head of the department of Greece and Rome.

Gradel now began a campaign of emails and letters hoping to stir the museum into action. It repeated its claims of nothing untoward, until October 2022, when a trustee forwarded one of the messages to the new museum chair, the former politician George Osborne. “I have taken your comments very seriously,” Osborne told Gradel.

The following July, Higgs was sacked. In August the museum’s director, Hartwig Fischer, resigned, and Jonathan Williams, the deputy director, voluntarily stood down and in due course also left the museum, which has brought a civil case against Higgs. He denies any wrongdoing.

Poor record-keeping had facilitated the thefts, and the British Museum soon announced a five-year plan to document online its entire collection of a nominal eight million objects, a formidable challenge. To date, of 1,500 missing or stolen pieces, 650 have been returned, and more are expected to follow.

The saga was covered by the BBC in 2024, in a Radio 4 series Thief at the Museum and a BBC Two film, both presented by Katie Razzall and featuring extensive interviews with Gradel. In the same year, Apollo magazine made Gradel its personality of the year.

Gradel was born in Haifa, Israel, to a British father and Danish mother. They settled in Nyborg, Denmark, when he was aged two. After leaving school he moved to London, where, between part-time working for the London Underground and a fish-and-chip shop, Gradel spent all his spare hours in the British Museum, later claiming to have seen practically every displayed object.

He returned to Denmark to study classical archaeology at Aarhus University, receiving a gold medal for his thesis on Roman emperor worship. In 1995 he obtained a DPhil at Oxford University, publishing his research as a much-admired monograph, Emperor Worship and Roman Religion (2002). After a year as a visiting fellow at Brasenose College, Oxford, and a stint teaching at the University of Copenhagen, in 2005 he took up the post of associate professor of history and archaeology of the Roman empire at the University of Reading. Colleagues there remember him variously as hilarious, eccentric and exasperating.

Though he never regretted leaving academia, he retained the dogged determination of a research historian. On one occasion Sotheby’s marketed on his behalf a Roman gem in an 18th-century gold setting, with an estimate of £100,000-£150,000: his description of the lot included 5,000 words of notes and sources. When the gem failed to find a buyer, Gradel told friends he would bequeath it to a museum.

His last publication, co-authored with Arne Pedersen, was The Lost Novel of King Solomon and the Demons (2025), a monograph inspired by the discovery of an antique ring engraved with the message: “Solomon says: Watch out!”

Shortly before his death, the British Museum presented Gradel with a medal, and a message from the current director, Nicholas Cullinan, recognising his expertise and “passionate determination that wrongs should be righted”.

Gradel is survived by his wife, Hanne Lavér Hansen, who was studying classical philology at Aarhus University when they met, and by his older brother, Kim.