Roman and Anglo-Saxon jewellery and other artefacts are still being sold illegally on eBay, despite the website’s promise to clamp down on the trade.

The British Museum has told The Times that it is alarmed at the number of sellers offering gold and silver that has apparently been found on British soil but has not been reported.

The Treasure Act 1996 requires the reporting of all gold and silver objects more than 300 years old, and groups of coins that are more than 300 years old and found on the same site.

Two years ago The Times reported that the number of potential treasure finds being offered for sale on eBay was so high that it was undermining the credibility of the Act.

This month the journal British Archaeology reports that between August and September this year almost 3,500 antiquities were offered for sale on the British eBay website, of which 600 were “British”.

In October eBay addressed the problem, signing a memorandum of understanding with the British Museum and The Museums, Libraries and Archives Council, the Government’s advisory body. The website promised to discourage the illegal trade in antiquities and agreed to allow the British Museum to contact sellers “to ascertain whether there is a reasonable cause for concern”.

But Claire Costin, of the Portable Antiquities Scheme at the British Museum, said yesterday: “The number of potential treasure finds for sale on eBay has not noticeably decreased since we began the monitoring process.”

Sellers have found a way round the problem. On being contacted by the British Museum, they simply insist that the objects came from overseas. Miss Costin said: “Frequently we are told that an item was bought abroad or was from an old collection, in which case there is not much that we can do, although in some cases we will inform the seller that they should provide evidence to buyers that the object has been legally exported from its country of origin.”

Among the recent offerings on eBay was an early medieval gold pendant. When contacted by the British Museum, the seller said that it had been bought at an antiquities fair in Germany in the 1980s.

The owner of a Roman silver bracelet said: “This bracelet originates from Italy. All of my items are from private collections of Central Europeans.”

The seller of a medieval silver ring became agitated after being contacted by the British Museum, saying in an e-mail: “I have responded and answered all your questions. I have been polite and courteous. I would now appreciate you cease this harassment and deal with eBay.”

A spokesman for eBay said: “If people are saying they don’t know where something’s from, then that is the truth, as far as we know.”

As many of the finds are sold by dealers or another third party rather than the finder, there is little information about whether such finds have or should have been reported.

Miss Costin said: “Many sellers also feel that they have no obligation to report the artefacts they are selling, feeling that this is the finder’s responsibility.

“We would love a change in the law to expand the obligation to report. We want all those who are involved in the sale of potential treasure artefacts to ensure that the finds have been reported, and also to be aware of the fact that finds should not be bought or sold unless evidence can be provided that the object has been disclaimed by the Crown.” The Treasure Act confers a legal obligation on all finders of treasure to report them to a coroner within 14 days of making the find, or realising the find was treasure. The penalties include imprisonment for up to three months and a £5,000 fine.

Michael Lewis, the deputy head of the British Museum’s Portable Antiquities Scheme, said: “People are buying treasure objects from whoever and not asking questions, then selling it on. We’d like illicit sales of antiquities to be stopped... Most antiquities have no provenance. They can be suspicious, but there’s no proof there’s anything dodgy.”

It means that newly discovered treasure items are being sold off without experts being given a chance to study them. Important information about our heritage could be lost.

The eBay spokesman said that if they did not sell the antiquities, people would find a way to sell them “one way or another”. He added: “eBay is the safest place to do so.”

Miss Costin said she was hopeful that things would improve. “It is still early days. Given that we have persuaded several eBay sellers to report artefacts — the majority of whom were previously genuinely unaware of the treasure Act and the legal obligation to report potential Treasure finds — and that these sellers have been very co-operative, we are hopeful there will be a long-term impact of our work.”

Auction site is a treasure trove for collectors - but have these items been reported?

• What eBay advertisement said: “Very rare silver Saxon ring of interwoven silver strands which do not normally survive because of fragility”

  What the British Museum said: “This was found in Cambridgeshire. If the finder thinks it is (Saxon) he had an obligation to report. He says it was found ‘some years ago’, which may suggest it should have been reported treasure trove rather than under the Treasure Act”

• “Rare Anglo-Saxon gold snake-ring, c6th century, found with a detector, Suffolk”

  British Museum: “The seller says object has been reported, but we would be interested to know that for sure”

• “Silver button, possibly medieval, found in Norfolk”

  British Museum: “Since the finder says this is medieval — though I think it might be more recent — it should be reported. The finder says he is going to take it to our finds officer in Suffolk (where he lives). If it is potential treasure he will remove it from sale and report it”