The work, which depicts Biblical heroine Judith beheading an Assyrian general, was found by the owners of a house near Toulouse as they investigated a leak.
It could be worth 120 million euros ($137 million), the Eric Turquin art expert agency said in a statement.
The painting is thought to have been painted in Rome in 1604-1605 by Michelangelo Merisi, known as Caravaggio, and is in exceptionally good conditions, Eric Turquin said, despite having been forgotten in the attic for probably more than 150 years.
"A painter is like us he has tics, and you have all the tics of Caravaggio in this. Not all of them, but many of them - enough to be sure that this is the hand, this is the writing of this great artist," Turquin told Reuters TV.
The owners of the painting had no idea they had it until they went to the top of the house to check a leak in the roof, Turquin said.
"They had to go through the attic and break a door which they had never opened .. They broke the door and behind it was that picture. It`s really incredible," he said.
French authorities have put a bar on it leaving France, describing it in a decree as a painting of "great artistic value, that could be identified as a lost painting by Caravaggio".
120 million euro `lost Caravaggio` discovered in French attic
13 April 2016
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A painting found in the attic of a house in southwest France two years ago was attributed to the Italian master Caravaggio by private French experts who hailed its discovery on Tuesday as a great event in the history of art.
