"It's very suggestive, very interesting, but the hard work remains to be done," said Martin Kemp, Professor of the History of Art at Oxford University and author of books on Leonardo.

"He (Menzies) says something is a copy just because they look similar. He says two things are almost identical when they are not," Kemp said.

"It's not strong on historical method," he added. But Kemp said he would look out for any signs that Leonardo had access to Chinese material, directly or indirectly, when studying his manuscripts in future.

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"I will keep my eye open, without thinking it is going to turn Leonardo studies or any studies of 15th century technology upside down."

Kemp said the source of the claimed Chinese influence was a separate issue.

"There is a whole series of questions a historian would ask about mediaeval technology, about Islamic technology, about transmission across trade routes, the Silk Route in particular.

"It's a terrifically complicated area and having a Chinese person in Florence in 1434 ends up giving that person a hell of a lot of work to do."

Menzies bases his claim that a Chinese ambassador went to Florence on a copy of a letter dated 1474 by Italian mathematician Toscanelli found among Columbus's papers.

Menzies publishes a translation from the letter reading: "In the days of Pope Eugenius there came a Chinese ambassador to him," although this is not explicit in the original Latin text.

"It's drivel", said Felipe Fernandez-Armesto, a British expert on maritime exploration who is a professor of history at Tufts University in the United States and at Queen Mary College, University of London.

"No reputable scholar would think that there is any reason to suppose that the person referred to by Toscanelli was Chinese," he told Reuters.

Geoff Wade, a senior research fellow at the National University of Singapore's Asia Research Institute, said Menzies' book and theories should be reclassified as historical fiction.

"Certainly Chinese ideas came to Europe and European ideas went to Iran and onwards," Wade said in a telephone interview.

"But the premise of the book that there was a Chinese fleet in 1434 which went to Italy is completely without any substance.

"There is absolutely no Chinese evidence for it."

Menzies brushes off the criticism, pointing to shelves of files in the rooms of his basement study filled with material he says supports his theories, much contributed by readers of his books and associated Web sites.

"I say the claim that critics make that there is no evidence is absolute rubbish. There is stacks and stacks of evidence.

"It's not me that's the fantasist, it's the historians who persist in this complete rubbish which is currently taught as history."