The heirs of Erich Lederer, who escaped to Switzerland, want Gustav Klimt’s famous Beethoven Frieze returned to the family.
The 1902 work is an experimental series of frescoes, which is 34 metres long and weighs four tonnes. It depicts an existential struggle for happiness, and is a homage to Beethoven’s Ninth Symphony.
It was criticized in the early 20th century for its strong eroticism, as it features floating female figures depicting wantonness and lasciviousness.
Since 1986 it has been on display at the avant garde Vienna Secession building - and is a huge draw for tourists and art lovers.
After WWII Lederer spent decades fighting Austrian officials for the right to take the Frieze to his home in Switzerland.
He gave up in 1973 and accepted $750,000 for the Frieze - though its assessed value was around $2 million.
The challenge by Lederer’s heirs is based on a 2009 change to Austria's art restitution law to cover cases in which the disputed art is sold for less than its value.
The Secession museum has said Lederer sold the frieze for a price “voluntarily negotiated by him and considered by him to be reasonable”.
Austrian officials have been forced to return a dozen stolen Klimt masterpieces since the 1990s.