Dali is regarded as one of the most important artists of the 20th century

In a vanishing act echoing the film The Thomas Crown Affair, the picture was stolen from its usual position and replaced with a replica - despite a 24-hour guard in the lobby.

Staff at the men's prison discovered the theft of the picture, which depicts Christ on the cross, on Saturday morning.

The original, last valued at $175,000 back in 1985, was drawn in red and black ink for a former Corrections Department commissioner and for many years hung in the inmates' dining room.

Fake spotted

"It's a great mystery at this point. It looks like the painting has been replaced by a copy," Thomas Antenen, a spokesman for the Department of Corrections, told the New York Times.

According to the New York Post, one of the prison's officers noticed that the image in the locked display case where the picture was housed "didn't look right".

It's difficult to imagine how a painting of this size would be removed from the jail Dali friend

Several more officers examined the picture and, having drawn the conclusion that something was amiss, called the police.

Investigators are baffled as to how the large painting - which measures three feet by four feet (one metre by 1.2) - could have been removed unnoticed.

In fact police are unable to even pinpoint when the painting could have been stolen and replaced by the fake, which was stapled to the back of the display case.

Gift from artist

Nico S Yperifanos, a close friend of Dali's, who delivered the painting to Rikers Island 38 years ago, said he thought the audacious plan must have involved several people.

"My god. It's difficult to imagine how a painting of this size would be removed from the jail. I don't know how it could have been removed intact," he told the New York Post.

Prisoners do not have access to the lobby, which is used only by prison personnel.

Experts have said that the painting could be worth at least three times its last estimate of $175,000.

Mr Yperifanos added that Dali, who died in 1989, would have been devastated by the theft.

Dali drew the sketch in 1965 as an apology to Anna Moscowitz Kross, the then Corrections Commissioner, after he was unable to attend a talk on art for the prisoners at Rikers because he was sick.

He dashed out the picture in just two hours and despatched Mr Yperifanos to deliver it.

On the corner he had written "For the dinning room [sic] of the Prisoners Rikers Ysland - SD".

And there it hung for 16 years until it was re-housed in the jail's lobby.