The head of the World Jewish Congress warned a Swiss art museum that it risks an “avalanche” of lawsuits if it accepts the bequest of a collection of artwork amassed by a man who dealt in art for the Nazis.

The Bern Art Museum discovered in May it had been named sole heir of Cornelius Gurlitt, the reclusive son of a man who dealt in so-called “degenerate” art for Adolf Hitler. The Bern museum has yet to decide whether to accept the artwork.

World Jewish Congress president Ronald Lauder said that since Gurlitt’s father, Hildebrand, had collected art stolen by the Nazis from Jewish collectors or taken from German state museums, Bern would have a problem on its hands if it accepted the works before their provenance has been fully investigated.

“If this museum in Switzerland gets involved with this inheritance, it will open Pandora’s box and unleash an avalanche of lawsuits - possibly from German museums, but certainly from the descendants of the Jewish owners,” Lauder said.

“The people in Bern will harm themselves and their country if they take these paintings before their provenance is cleared up. They would become a museum of stolen art,” he told German magazine Der Spiegel in an interview to be published on Sunday.

Gurlitt died in May at the age of 81, in the flat in Munich where he lived and stored the art collection. The Bern museum said news of his bequest came “like a bolt from the blue,” because it had not had any connection with him.

Hundreds of masterpieces by the likes of Chagall and Picasso were secretly stored by Gurlitt at the Munich apartment and a house in nearby Salzburg, Austria. He occasionally sold pieces to finance his quiet lifestyle and his healthcare. The collection is worth an estimated 1 billion euros ($1.4 billion).

The Gurlitt family had said its collection was destroyed in the bombing of their home in Dresden during World War Two. Its survival remained secret until 2012, when tax inspectors stumbled across the hoard during an unrelated inquiry.

The Bern museum denied a German media report last month that it had decided to accept the artworks. It said it was still in talks with German authorities to ascertain all the implications of accepting the inheritance.

“In the end our board of trustees is free to decide whether it is in the best interests of the Bern Art Museum to accept or decline the estate,” it said in a statement in mid-October.