Britain's Guardian newspaper said the cache discovered three months ago is conservatively estimated at $79 million.
The works, many of which were previously unknown, date from 1900 to 1932.
An elderly man calling himself Pierre Le Guennec brought the pieces to the offices of the Picasso Administration and told the 20th century artist's son, Claude, the famed painter and his wife, Jacqueline, had given them to him after he installed alarm systems at the painter's various homes during the three years before his death in 1973.
Doubting Le Guennec's story, the Picasso family has subsequently taken legal action against the man for receiving stolen goods and police are investigating the matter, the newspaper said.
"He would never have let go of so many pieces of his oeuvre in one go," Claude Picasso told the French daily newspaper Liberation. "To have given so much … it doesn't make sense, to be honest. It's true Pablo Picasso was quite generous, but he always dated, signed and dedicated his gifts, especially as he knew some would sell them to cover their needs."
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