UNESCO has thrown cold water over an American explorer's claims he has discovered the sunken treasure of infamous 17th-century pirate William Kidd off the coast of Madagascar.

Marine archaeologist Barry Clifford declared in May that he had solved an enduring mystery of the high seas by locating the wreck of Kidd's "Adventure Galley" ship and a 50kg silver ingot.

But a team from UNESCO, the United Nations' cultural body, visited the site to verify Clifford's claims and brusquely dismissed his highly-publicised announcement.

The UNESCO report said the "silver" ingot was just a lead weight, and that the supposed shipwreck was old rubble in a bay of Sainte Marie, a small island east of Madagascar.

"What had been identified as the Adventure Galley of the pirate Captain Kidd has been found ... to be a broken part of the Sainte-Marie port constructions. No ship remains have been found," the report said.

"Also the metal ingot, recovered apparently from the above site, is not a 'silver treasure', but is constituted of 95 per cent lead.

"It does not contain silver and has been identified as a lead-ballast piece."

Clifford, who is making a television documentary based on his hunt, garnered world headlines in May where he unveiled the "ingot" before Madagascan President Hery Rajaonarimampianina and the US and British ambassadors.

But UNESCO soon raised doubts about Clifford's work, and criticised him for not having a professional archaeologist supervising the dive.

The legendary Captain Kidd, who was born in Scotland in about 1645, was first employed by British authorities to hunt pirates, before he himself turned into a ruthless criminal.

He is thought to have scuttled the Adventure Galley, which was armed with 34 big guns, during an expedition to the Indian Ocean.

After looting a ship laden with valuable cargo in 1698, Kidd was caught, imprisoned and questioned by the British parliament before being executed in 1701.

The fate of much of his booty has remained a mystery, sparking intrigue and excitement for generations of treasure-hunters.

Clifford's team said it stood by its claims.

Documentary producer Sam Browne insisted the team had conducted "the most comprehensive geophysical study ever done" of the bay.

US explorer Barry Clifford says he believes there are many more such bars still in the wreck, BBC News reports. Pictured is the spot where the silver was found, just off the coast in shallow water.

The bar was presented to Madagascar's president at a special ceremony.