A trove of around 1,500 paintings worth nearly £1bn, by artists including Picasso, Matisse and Chagall, have reportedly been uncovered in a Munich apartment.
The collection of paintings had been confiscated by the Nazis and up to 200 had been the subject of international warrants.
The German magazine Focus said the trove included works by Pablo Picasso, Henri Matisse, Marc Chagall and Renoir.
One of the paintings is said to be a portrait of a woman by Matisse that belonged in the collection of the Jewish connoisseur Paul Rosenberg, who had to leave behind his collection before his escape from Paris.
According to the magazine, the artworks were found in early 2011 when tax authorities investigated the reclusive son of an art dealer in Munich, called Cornelius Gurlitt.
After inspectors obtained a search warrant for his home in Munich, on suspicion of tax evasion, they found the stash of 1,500 artworks.
He had sold paintings when he needed the money. It is reported that Mr Gurlitt never worked and had no other means of income. He was described by a German official as "a man who did not exist" - he had no pension and no health insurance.
The artwork was said to have been found by a mound of tinned and bottled food.
The raid, however, did not come to light at the time as customs placed a ban on information about it.
The Nazis confiscated much modern art, which they labelled "degenerate", and was banned for being un-German or for being the work of Jewish artists.