The origins of the metre-tall marble bust of the Father of the Gods have been shrouded in mystery for decades.

Now in the care of English Heritage at its archaeological store in Helmsley, the newly identified work is about to begin a long process of conservation.

Before then it will be put on show as one of the highlights of a series of free tours at the facility, which is normally off limits to the public, starting on April 21.

The bust was probably brought to England from Italy and became part of the Earl of Arundel's famous collection of antiquities from the classical world. It was first documented in 1616. The collection was dispersed in the 17th century, but 100 years later the bust fell into the hands of MP John Aislabie, the owner of Fountains Abbey and Studley Royal, near Ripon. It may have taken pride of place in the Temple of Jupiter on the Studley Royal estate, adding a touch of authenticity to the 18th-century folly.

The find has been verified by Professor Michael Vickers, of Oxford University, and senior assistant keeper of The Ashmolean Museum, Oxford, which holds the largest collection of Arundel marbles.

English Heritage curator Susan Harrison said: "The condition of the bust is pretty good but it does need cleaning and further research. Because it's mentioned in 17th-century records experts knew it existed, but they have searched for it in vain among the world's collections of antiquities. But here it was all along, safe and sound and waiting to be rediscovered. We plan to do further work to determine the type of marble which will give us an indication of where in the roman world it is originally from.

"It really is an impressive piece."

According to Roman mythology Jupiter was the most powerful of the gods and the Latin depiction of Zeus. Such busts were worshipped throughout the empire before the official conversion to Christianity proclaimed by Constan- tine the Great, who was made Emperor in York in 306AD.

The Helmsley store is home to more than 800,000 artefacts from prehistoric flints to 20th-century church fittings, and nowhere else in Western Europe has as much carved medieval monastic stone.

Access is given to scholars from across the world.

This year's free public tours will also showcase weapons including arrowheads and horse spurs from Norham Castle, Northumberland.

Tours, which are wheelchair accessible, take place at 11am, 1pm and 3pm on April 21, May 26, June 23, July 21, August 25, September 22 and October 20.