The painting has attracted a circle of people who are trying to prove it's a fake

National Gallery spokesman

The magazine has reprinted theories that the work was actually painted by Van Gogh's friend, artist Paul Gauguin - copying a Van Gogh original.

But the National Gallery in London - where the painting used to hang - has disputed the claim.

"The version of the Sunflowers belonging to the Yasuda Fire and Marine Insurance Company hung on our wall for more than 10 years and its authenticity was never questioned," a gallery spokesman told BBC News Online.

However the article's author Antonio de Robertis says that between 1888 and 1889 Gauguin tried to borrow one of the original Sunflowers but was refused.

'Valuable assistance'

Gauguin, de Robertis believes, then borrowed the painting "on the quiet" and made his own copy.

Quadri e Sculture has said that their investigation was supported by "valuable assistance" from London's National Gallery.

But the National Gallery told BBC News Online that it has had no contact with the magazine.

A National Gallery spokesman said: "Since the painting sold for an astronomical price it has attracted a circle of people who are trying to prove it's a fake.

"The picture will be reunited with our Sunflowers, and the Sunflowers from the Amsterdam Van Gogh museum, in an exhibition there this winter.

"This will be the first time they have been seen together in a long time and people will be able to see that the Yasuda Sunflowers is fine."