Heirs of the former owners are challenging the Russians to return them and, though no official figure has been disclosed, an estimate of their value would be around £200m. "Ownership of the paintings is a political matter," Professor Mikhail Piotrovsky, director of the Hermitage, told me. "It will be decided by a vote in the Russian parliament or by presidential decree."
Prof Piotrovsky, 50, was appointed director by decree of the Russian Prime Minister in 1992 and has breathed new openness and dynamism into the museum. He has organised an exhibition of the "Hidden Treasures" which is scheduled to open on 30 March. "While they are entries on secret lists, the paintings are only symbols of money," he told me. "The exhibition will turn them back into works of art - which can be appreciated irrespective of their ownership."
The identity of the paintings is to be officially revealed to the world press at a conference in Catherine the Great's theatre, beside the River Neva in St Petersburg, next Thursday but Prof Piotrovsky gave me a personal preview last week.
Still unframed, the paintings were stacked against the walls of the studio in the Romanov's old Winter Palace. A restorer prop-ped each masterpiece in turn on a red plush stool for our appreciation - an unusual and moving way to view great art.
The star turn is Degas' Place de la Concorde of 1875 with the artist's friend, Vicomte Lepic, his daughters and a dog in the foreground. It is a triumph of balance between space and figures.
Two Gauguins, painted on the artist's first visit to Tahiti in 1892, run the Degas close. Piti Teina ("The Two Sisters"), a study of two young girls, is completely unknown.
There are four Van Goghs, the finest being a small landscape of ploughed fields centred by a yellow house with a red roof which he painted in 1889.
There are seven Cezannes, including a fine study of bathers, a host of Renoirs of varying quality, six Monets, a dazzling Seurat landscape, a sensuous nude by Courbet, an Absinthe Drinker by Picasso, and a portrait of a Montmartre prostitute, Berthe la Sourde, by Toulouse-Lautrec.
While treasures from German museums were transported to Russia in bulk in 1945, most of the material was returned to Berlin, Dresden and other East German institutions in 1958. The secret storerooms of the Hermitage contain what Piotrovsky calls "the leftovers". It is only in the past two years that the existence of these stores has been admitted. During our conversation last week, he revealed what they possess.
There are 700 paintings, mostly decorative Old Masters of no exceptional quality; Impressionist graphics from the same German collections as the oils; drawings from the Bremen Kunsthalle; a group of silver objects, which "we think" came from palaces nearDresden; a big collection of oriental art from the Ostasiatische Museum in Berlin; important archaeological material excavated by Schliemann from Troy; sculptures from "various places" and some medieval stained glass from the Marienkirche in Frankfurt-am-Oder.
Roughly 85 per cent of the Impressionist paintings come from the collection of a German manufacturer of boilers called Otto Krebs who lived in Holzdorf, near Weimar, and died in 1941. In 1945 his home was turned into a Soviet military headquarters and the paintings were removed to Russia. Krebs died childless but a medical foundation in Mannheim which he endowed, the Stiftung fur Krebs-und-Scharlachforschung, is claiming ownership of the paintings. Several have turned out to be fakes; a dud Toulouse-Lautrec nearly made it into the exhibition.
In contrast, the small group of paintings from the collection of Otto Gerstenberg, a turn-of-the-century insurance magnate, are all masterpieces. They include the Degas Place de la Concorde, three Renoirs, and a Daumier.
Gerstenberg died in 1935, leaving the collection to his daughter, Margarete Scharf. She gave the paintings to the Berlin National Gallery for safekeeping at the outbreak of war. The Russians found them in a bunker in the Zoological Garden, along with thegallery's own paintings. Margarete Scharf's sons, Dieter and Walter, have visited the Hermitage to see the family pictures and are trying to get them back.
Another small but distinguished group of pictures comes from the collection of Bernard Koehler, a businessman whose heirs are still untraced; there is a Degas that belonged to Friedrich Siemens; a Monet garden scene from Bremen Kunsthalle; two paintings inscribed "Alice Meyer" and one inscribed "Baron von der Heydt".
A special report on the Hidden Treasures, including new colour photographs, will appear in next week's `Sunday Review'.