Professor Martin Henig, a specialist in engraved gems, said he and a colleague turned detective after spotting part of an ancient signet ring, thought to be in the museum's collection, on sale with a private dealer.

The museum was alerted and retrieved the ring, thought to date from the time of the first Roman emperor Caesar Augustus, but the academics discovered more items for sale, some on the online auction site eBay.

Prof Henig, from Oxford University, said: "My colleague tried to report this to the museum but it seemed it didn't get through to the right people. He got very upset and said it really needed to be stopped, but I don't know what serious investigation was going on.

"There was clearly a serious breach of small objects appearing and he rather suspected others were because he kept an eye on things like eBay."

Prof Henig said the ring was probably worth around £1,000 and its discovery should have triggered a major investigation at the museum.

He said: "Bells should have been running right up to the top, but it may well have been taken as being more minor because I suppose occasionally people wander off with an odd thing from museums. It's often a member of the public, somebody like that, who's drunk."

Neither the museum, nor Scotland Yard which is investigating the theft, has said how many items are missing or their value, but some - gold jewellery, semi-precious gems and glassware - date back to around three and a half thousand years ago.

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"The number of items must be quite large because it seems to have been going on for a long time," said Prof Henig.

"I was told there was a junior curator who was in tears because what seems to have happened is that some of the things in order to make that more saleable and less recognised, say a precious object, a gem or something like that in a gold setting, if you take it out of the gold setting, then you can melt down with gold. It has just a scrap value. I think there was a lot of damage done."

In the wake of the thefts, the museum's curator of Roman cultures Peter Higgs was fired earlier this year. He had worked for the museum for 30 years.

He has not spoken about his dismissal, but his son Gregory told The Times newspaper his father had not stolen anything and had been badly treated.

Scotland Yard said it is investigating the thefts but has made no arrests.

Prof Henig said: "I find it so extraordinary that a scholar whose scholarship is unimpeachable, could be involved. I mean, it's a vacation working in a museum."

He believed the museum had tried to keep the matter quiet to avoid damage to its reputation as one of the world's most prestigious galleries with around eight million items of human art and culture.

"It seemed to me to be so similar to what has happened in the Church where a bishop covers things up because he doesn't want to have a scandal," he said.