"Following a thorough investigation it was identified as the front of a white marble sarcophagus, and is covered with extremely fine bas-relief carvings depicting a drunken Dionysus leaning on a satyr and flanked by 'party revellers' including Hercules and Ariadne and two large lion heads," the palace website stated.
An antiques expert visiting the palace on unrelated business was responsible for identifying the sarcophagus after noticing the white marble front.
Nicholas Banfield of Cliveden Conservation oversaw the restoration of the coffin, which measures 6 feet long and weighs about 881 pounds, and said it was in "remarkable condition" despite spending so much time out in the elements.
"Following an initial in-situ inspection we were able to unbolt it from the lead cistern to which it was attached and take it back to our workshops for full cleaning, repair and stabilization," he said.
Palace spokesman Jonathan Prince told the New York Times similar coffins have sold for as much as $120,000 at auction, but said the sarcophagus, which is valued at $364,590, will remain on the property.
"But it is not for sale, not at all," he said. "It will stay here."