HIDDEN parts of a historic jewellery chest have been unlocked for the first time in decades.

A missing key meant sections of a rare 18th-century coffer at Blenheim Palace have been shrouded in mystery for generations.

But staff at the World Heritage Site in Woodstock, West Oxfordshire, have finally been able to see into the object's secret compartments.

The Louis XIV coffer, used to hold jewellery and other precious items, had been undergoing wider restoration work after parts of its veneer began peeling away.

Blenheim's house manager, Kate Ballenger, said: "For as long as anyone can remember, the upper section of the coffer had remained locked, as the key had been mislaid.

"When the restorer was able to finally open the upper lid of the coffer, he discovered a series of secret compartments hidden behind hinges, where items of particular value could be concealed."

The coffer was designed in the late 17th century by a skilled French artisan called André Charles Boulle.

He was awarded the title of cabinet-maker and sculptor to King Louis XIV of France in 1672, and his pieces are displayed in museums from Paris to New York.

Ms Ballenger said: "The newly-opened upper section of the coffer reveals Boulle's extraordinary craftsmanship in its original state – although sadly all the secret drawers were empty."

She said the chest is believed to have been among items bought by the 9th Duke of Marlborough after his marriage to Consuelo Vanderbilt in 1895.

It was given its striking design through a process called marquetry, in which small pieces of material – usually wood – are inlaid to make decorative patterns.

Boulle marquetry in particular involves sandwiching together thin sheets of brass, pewter and tortoiseshell and cutting out designs using an intricate saw.

Explaining reasons behind the restoration, Ms Ballenger said: "Due to the process and materials used, it is very fragile and susceptible to environmental change over the centuries.

"As a result some sections of the marquetry had begun to separate and we took the decision to bring in a specialist restorer."

She said the restorer, Ludovic Potts, was able to 'unpick' the locked section of the coffer.

Blenheim did not put a price on the prize possession, but said a similar pair of coffers sold for more than £2.6m in 2009 at Christie's auction house.

Its coffer is the same model of a pair known as 'du Grand Dauphin', which are located in the Getty Museum in Los Angeles.