Ronald Lauder, head of the World Jewish Congress, has warned a museum in Switzerland against accepting the collection it inherited from reclusive German art collector Cornelius Gurlitt, who died in May this year.
In an interview with Der Spiegel magazine published Saturday, Mr Lauder said the Kunstmuseum in Bern would risk "opening Pandora's box, triggering an avalanche of lawsuits".
Mr Lauder was interviewed together with Germany's Minister of Culture, Monika Grütters, who confirmed that the German government was holding talks with the museum director.
She agreed with Mr Lauder that German museums generally do not do enough to find the rightful owners of ill-gotten art, although the government has increased funding for research into the provenance of such works.
The Swiss museum is to decide later this month whether it will accept the controversial Gurlitt trove, which was uncovered during a 2012 probe into his tax affairs. It includes works by such greats as Max Beckmann, Marc Chagall, Pablo Picasso and Henri Matisse.
News of the discovery broke a year ago, thanks to a Focus magazine exposé. Stashed in Gurlitt's Munich apartment were hundreds of works by prominent artists.
Research into their provenance is ongoing, but many of the works were probably forcibly taken from Jewish collectors during the Third Reich. Gurlitt's father, Hildebrand, was an official collector on assignment for the Nazi government; he apparently bought up works that had been confiscated or purchased under duress.
The find triggered the establishment of a commission in Germany to help research the provenance of the works. Restitution of some paintings to heirs of original Jewish owners had already begun when Gurlitt died and left the collection to the Bern museum.
Should the museum accept the collection, it would also inherit the difficult and expensive task of establishing the legal owners of all the works.