But the city mayor, Jose Maria Alvarez de Manzano, a deeply conservative man with no sympathy for the devil, insists Lucifer belongs to the people of Madrid and will not let it be moved from its present site.
The Prado's claim is part of a controversial attempt to regroup 126 of more than 4,000 works dispersed throughout Spain into its enlarged premises, due to be completed in 2001. But with more than 90 per cent of the museum's treasures stored in vaults out of public view, critics say the museum has enough material already, without plundering much-loved public monuments.
Antonio Solano, a museum curator, said: "The sculpture of `El Angel Caido' belongs to the museum. It was destined for the former Museum of Modern Art, which now forms part of the Cason del Buen Retiro. But as there wasn't room for it, it was lent as an adornment for the Retiro. Now we have decided to remind the city hall that it belongs to the Prado."
A director of monuments at Madrid's town hall denies the image of the devil is the Prado's: "According to our archives, it was donated to the city hall by the Ministry of Works in 1885 and it was decided to put it in the Retiro. Furthermore, it is entered in the inventory of Spanish Historic Heritage of 1992 as municipal property. I don't understand why the Prado is claiming it now. It's always been in the park. What do they want it for?" Mr Alvarez de Manzano has written to the Prado insisting that while he is mayor, the statue stays where it is.
The sculpture, by Ricardo Bellver, stands without any identifying label in the bleaker south-west corner of the Prado, on a site formerly occupied by a porcelain factory.
Nothing more sinister than rollerbladers circulate around an area of the park reputed to be a night-time cruising ground for transvestite prostitutes - but legend has it that nocturnal black masses used to be held in the Satanic presence. Some years ago a failed attempt was made to erect a statue of the Virgin to counteract Lucifer's supposed malevolent influence.
The determination of the city authorities to keep their unique tribute to Satan seems to have had an effect: in recent days, the Prado has said that although it owns the statue, it does not necessarily seek to possess it, leaving open the prospect that the people of Madrid will hold on to the devil they know.