The treasure trove of 271 pieces includes lithographs, cubist paintings, notebooks and a watercolour and is said to be worth about 60m euros (£50.6m).
Pierre Le Guennec, 71, reportedly says Picasso gave him the works as gifts.
But the estate’s administrators have filed a case for alleged illegal receipt of the works of art.
According to French newspaper Liberation, the lost Picassos include a watercolour from his Blue Period. Experts say the nine cubist works in Mr Le Guennec’s possession are worth 40m euros alone.