Previously unknown masterpieces by modernist painters Marc Chagall and Otto Dix are among the vast trove of works believed stolen by the Nazis and hidden in a Munich flat, an art historian says.

Meike Hoffmann, an expert aiding the investigation, told a packed press conference the Chagall painting, an allegorical scene dating from the mid-1920s, had a “particularly high art-historical value”. The Dix work is a rare self-portrait believed to have been painted in 1919.

Hoffmann showed slides of the paintings, which also include works by Pablo Picasso and Henri Matisse, at a news conference in the southern city of Augsburg where the German authorities shed light on the spectacular find in the apartment of an eccentric elderly loner.

The man, identified as Cornelius Gurlitt, is the son of Hildebrand Gurlitt, a prominent Nazi-era art dealer who acquired the paintings in the 1930s and 1940s.

Hildebrand Gurlitt had been one of a handful of art experts tasked by the Nazis with selling valuable artworks stolen from Jewish collectors or seized among avant-garde works deemed to be “degenerate”.

Augsburg chief prosecutor Reinhard Nemetz said 1285 unframed and 121 framed paintings, sketches and prints were found in the rubbish-strewn flat, some dating back to the 16th century.