The billionaire Franco-American art dealer, Guy Wildenstein, has been formally accused of fraud after 30 valuable paintings and sculptures that had been missing for decades were discovered in the strong room of his family's institute in Paris.

Mr Wildenstein, 65, a leading financial backer and friend of President Nicolas Sarkozy, blamed the presence of the works, including several allegedly looted by the Nazis, on "an oversight" by his late father. He has been placed under formal investigation by a French magistrate on a possible charge of "receiving fraudulently obtained goods".

The case is part of a cat's cradle of interconnected lawsuits, on both sides of the Atlantic, which threatens one of the wealthiest and most powerful art-dealing families in the world. Mr Wildenstein is the son of Daniel Wildenstein, who made his reputation and fortune, from cataloguing and selling the works of French impressionists, including Claude Monet and Édouard Manet.

The missing works were first discovered during a police raid in Paris at the end of last year. The investigators were looking for priceless artworks belonging to Daniel Wildenstein, which his second wife, Sylvia, believed to have been hidden away by her two stepsons after his death in 2001.

Instead, officers from the French agency which tracks stolen art – L'Office Central de lutte contre le trafic des Biens Culturels – found 30 works from other large French family collections which had been missing for decades.

They included Cottage en Normandie, by the impressionist artist Berthe Morisot, whose whereabouts had been in dispute since the 1990s. They also included sketches and sculptures by another impressionist, Edgar Degas, which were believed to have been looted by the Nazis from a mansion near Paris in 1941.