UNESCO on Tuesday threw cold water over an American explorer's claims that he had 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 50-kilogramme (110-pound) 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% lead.

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

Clifford, who is making a television programme based on his hunt, garnered world headlines in May where he unveiled the "silver ingot" in front of 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 of the high seas.

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.

This is the second time UNESCO has clashed with Clifford.

Last year, he claimed he had identified the wreck of Christopher Columbus's flagship that sank in 1492 off the northern coast of Haiti.

The claim was soon disproved by UNESCO, which determined it was a ship from a later period.

Clifford is best known as the discoverer and excavator of the world's first fully verified pirate shipwreck, the Whydah, in 1984.

Sainte Marie is a haven of pirate lore, attracting treasure-hunters and shipwreck enthusiasts from around the world.