Thieves stole a valuable painting by 19th century artist Edgar Degas overnight from a French museum, police said yesterday.

The colourful image of singers performing on a theatre stage was missing when staff opened up the Cantini Museum in the southern port city of Marseille, prosecutor Jacques Dallest said.

The national museums service said the picture was a pastel work titled "The Chorus", worth €800,000, correcting an estimate given by police and museum staff that it was worth some €30 million.

Loaned by the Orsay museum in Paris for an exhibition of theatre-themed artworks including some 20 works by Degas, it measures 32 by 27 centimetres. Mr Dallest said it had been unscrewed from the wall.

"As far as I know there was no break-in," Dallest said, adding that investigators suspected an intruder, a visitor to the exhibition or an inside job.

City councillor Maurice Di Nocera, responsible for organising major events in Marseille, called the theft "a disaster for the museum."

The museum was closed yesterday while police investigated, examining film from security cameras.

The city hall, which runs the museum, said 70,000 people had visited the exhibition and the show was due to close in Marseille on January 3 and go on tour later to Italy and Canada.