The Filippo Negroli helmet, thought to date back to the 1530s, was the most expensive single item of armour in the world when it was sold in July 1997. But experts brought in by the museum in Leeds found that it is likely to be the work of an unknown 16th century artisan imitating the man acknowledged as the greatest armour embosser of all time.
The Armouries spent nearly a third of its pounds 350,000 annual acquisitions budget on the intricate parade helmet.
Claude Blair, a leading expert in the field, said: "The Armouries made a blunder. This is a very dubious item and I condemn them for not consulting widely enough."
Mr Blair, the former keeper of metalwork at the Victoria and Albert Museum in London, said he felt the museum rushed into the purchase after it failed to get funding to buy the armour of Henry II of France, made by Giovanni Paoli Negroli, a close relative of Filippo. "There has been doubt about the authenticity of this piece for years. When you compare it to other Negroli pieces it is not in the same class. It may be an imitation made by someone minor around that time. It may be a fake. But I have no doubt it is not by Negroli," he said.
Auctioneers Sotheby's stood by its claim that the helmet was a Negroli but said that there was dispute over its authenticity.
The helmet was thought to be one of only three of Negroli's helmets on public display. It is now in storage. It was sold by the Earls of Warwick.
Guy Wilson, Master of the Armouries, said the helmet had been subject to "considerable research" since it was bought. It involved taking the helmet to New York in 1998 for comparison with comparable Negroli pieces. "The international experts that were present for the comparison were of the opinion that it was not the work of Filippo Negroli but that it may be by another Milanese armourer of the 16th century working in imitation of Filippo's style. We are currently in discussion with Sotheby's about the implications of this discovery," he said.
A Sotheby's spokesman, Christopher Proudlove, said: "We stand by our original attribution that this was made by Filippo Negroli. What we are talking about is a difference of opinion, whether Filippo Negroli made this or another of his scholars or people who worked with him. The only problem is with attribution. There is no shadow of a doubt that this is 16th century piece. It is certainly not a fake." He added that Sotheby's guaranteed items sold for five years.