James Ede, an antiquities specialist and member of the Government’s Advisory Panel on Illicit Trade, realised that it was a treasure that had gone missing from the Archaeological Museum of Thebes as he began researching the piece.

He found that the stele, dating from the 1st century and valued at about £20,000, had been referred to in publications as part of the Thebes collection since 1923. “At that point, I got on to the Greeks,” he said.

The stele had been stolen in 1981, it emerged. No arrests have been made. It will be handed to Evangelos Venizelos, the Greek Minister of Culture, on Monday while he is visiting London.

An anonymous seller in Paris consigned the stele to Bonhams, where it was bought by another London dealer. He in turn sold it on to Mr Ede at this year’s Maastricht art fair.

Bonhams has reimbursed the dealer, who has also paid back the money to Mr Ede, who declined a reward from the Greeks. He would be entitled to a high percentage of its value. “It is not appropriate,” he said yesterday. “I haven’t lost any money and the piece is going back to where it should be.”

He said that the stele bears a list of graduates of a gymnasium, in effect an old boys’ board from a school, with up to 20 names under the heading “good fortune”. Such pieces do not appear on the market often, he said. “They generally look like tombstones. They’re not objects of enormous beauty, yet they can be striking.”

Mr Ede, who is vicechairman of the International Association of Dealers in Ancient Art and a member of the committee for the Antiquities Dealers’ Association, said that Bonhams acted in good faith, checking the Art Loss Register’s database of stolen art. “It is very unfortunate that the piece was never reported as stolen.”

Victoria Solomonidis, the cultural attaché at the Greek Embassy in London, said: “It is great that James Ede volunteered to return it to Greece, not only because the stele belongs to a particular collection and it has to be seen and studied as part of that collection, but also because he is setting a great example for antiquities dealers around the world.”

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