Vienna's art museum is bracing for a protracted legal battle with the heirs of a man who reportedly sold his Vermeer painting to Adolf Hitler in 1940.

The Austrian Culture Ministry revealed earlier in September that it has received a formal request for the restitution of The Art of Painting by 17th century Flemish master Johannes Vermeer, which has been displayed at the Kunsthistorisches Museum since 1946.

Sabine Haag, director of the museum, called it "an absolute jewel."

The work depicts a painter painting his female model, who is standing by a window in his studio, with a map of the Netherlands on the wall. It is considered a masterpiece, noted among other things for the way in which it shows the play of light upon surfaces.

The Art of Painting was a favourite of Vermeer, who refused to sell it in his lifetime.

Sold to guarantee family's safety

The family of Jaromir Czernin has been petitioning for its restitution since the 1960s but the Austrian government kept rejecting the plea because it said the sale was voluntary.

Now the Czernin family says it has a report commissioned by an expert who says the sale was made under threat.

"[Jaromir] had to sell it in order to guarantee his family's safety," Czernin family lawyer Andreas Theiss told Der Standard newspaper.

Czernin was the brother-in-law of Austrian chancellor Kurt von Schuschnigg, who was in power from 1934 to 1938, and he opposed Hitler's annexation of his country. As well, Czernin was married to a woman of Jewish heritage.

He sold the painting for 1.65 million Reichsmark.

Under a 1998 restitution law, Austria has returned approximately 10,000 works of art stolen by the Nazis after the country was annexed by the Nazis in 1938.

One of the most notable cases was the restoration of five paintings by Gustav Klimt by the Belvedere Museum to the descendants of the previous owner.

Fewer than 40 paintings by Vermeer are known to exist. The work could be worth tens of millions of dollars.