When the painting, signed 'A Hitler 1910', was sold amid much fanfare in Apr, the auctioneer's historical documents expert boasted about the amount of interest that it had received.
However, the same 'expert' now admitted that the painting might be a fake after Mandrake pointed out to him that it was, in fact, likely to be a painting of Clapper Bridge on Dartmoor, reports The Telegraph.
"It's impossible to know exactly if they are genuine," says Richard Westwood-Brookes, of Mullocks auction house, which sold the picture along with 12 other watercolours that it claimed were by the Nazi leader.
"It's like if someone gave you a piece of wood and said 'this is from the Battle of Trafalgar', you wouldn't be able to tell 100 per cent if it was. You can never be entirely sure.
"Nobody came forward despite a huge amount of publicity before the sale to say that the paintings were fake," he added.
Historian David Irving, who initially denounced the 'Hitler Diaries' that were published by The Sunday Times in 1983 as forgeries, says he "hopes the buyer has a money-back guarantee".
He says: "I warned all concerned that the paintings are all fakes. I have handled perhaps a score of genuine Hitler artworks, usually sold by the people, close friends, to whom Hitler had given them. They were all catalogued at one time."