MADRID, Spain — The most famous art museum in Madrid, the Prado, said it has “mislaid” a 19th-century stone sculpture weighing three tons that was supposed to form the centerpiece of a renovated wing.

The statue, which represents the three muses of painting, sculpture, and architecture, is the work of the Spanish sculptor Jeronimo Sunol and it had graced the entrance to the north wing of the Prado Museum since the late 1800s.

But it was removed for repairs and cleaning sometime in the 1970s and has not been seen since.

Architects in charge of renovating the facade of the building said they had tried to track down the piece, which they hoped to restore to its original place, but it failed to appear on any inventory records. “The niche that it sits in is there, ready and waiting,” said Rafael Olalguiaga, an architect involved in the restoration of the museum. “But it has just disappeared, as if by magic.” He said neither the Museum, the Ministry of Culture, nor the Madrid City Hall, which has stored large artworks in the past, have any record of the piece. According to auctioneers in Madrid, the work is priceless.

The last time that the muses were definitively recorded in situ was in the early 1960s, when art collector and painter Thomas Harris photographed the group from a balcony in the adjacent Hotel Ritz. Mr. Harris, a British secret agent during World War II, died shortly afterward in a car accident in Mallorca.

The loss of Sunol’s sculpture has come as another blow to the museum, which has been embroiled in controversy over the building of a much-needed extension that began in 2002 with a budget of $58 million and was expected to last two years.

The new annex, designed by modernist architect Rafael Moneo, will finally open to the public this October, following a succession of delays and legal squabbles that pushed the cost to more than $184 million.