Works by several of the greatest artists of the modern age have been held at a secret location near Munich for the past two years, after having been discovered in a raid by the German authorities on the apartment of an elderly man accused of tax fraud.

The 1,500 pieces include works by Picasso, Matisse, Chagall and Klee. According to the German magazine, Focus, which broke the story, their total value is in the region of 1 billion Euros.

The works are believed to have belonged to Jewish collectors and considered "degenerate" before World War II and to have been confiscated by the Nazis. Many were considered lost until now. They were purchased during the 1930s and 1940s by the German art dealer Hildebrandt Gurlitt, who was an associate of the Nazi leadership.

Gurlitt's son Cornelius, 80, kept them for half a century in a dark room in his apartment, following his father's death in 1956. He is believed to have sold off some of the works to support himself.

Cornelius Gurlitt was stopped during a routine check by the tax authorities in 2010, while returning to Munich from Switzerland. The check led to the launch of an investigation for tax evasion and a search of Gurlitt’s home in 2011, during which the works were discovered.

Investigators told the German media that Gurlitt was a recluse, and did not resist the raid or the confiscation of the art.

The discovery has been kept from the public until now for fear of a diplomatic and legal struggle to restore them to their rightful owners and heirs, according to the website of the weekly Der Spiegel.

According to an examination by Berlin art historian Meike Hoffmann, at least 300 of the works were deemed “degenerate” by the Nazis.

After the raid but before the artworks were seized, Cornelius Gurlitt sold a piece by Max Beckmann named "Lion Tamer, Circus" for 864,000 Euros via a Cologne auction house, Focus reported.

One of the best pieces in the collection is said to be "Portrait of a Lady" by Matisse, which once belong to French collector Paul Rosenburg. Rosenburg, who lost his collection when he fled for his life during the fall of Paris, was the grandfather of Anne Sinclair, the former wife of the disgraced head of the International Monetary Fund, Dominique Strauss-Kahn. Miss Sinclair has been a prominent campaigner for the return of art looted by the Nazis to their former owners.

Open gallery view Cover of Focus magazine that broke the story