The Most Noble Order of the Garter, a Lesser George Sash Badge, with central agate cameo of St George and the Dragon by Benedetto Pistrucci set in a 22ct gold and blue enamel mount by William Clutton. Estimate £100,000-150,000 at Sotheby’s.

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On July 3, Sotheby’s – in conjunction with the Morton & Eden saleroom – will offer royal orders and medals from the collection of George, Duke of Cambridge (1819-1904), George III’s grandson.

This group has been passed down by direct descent. They are primarily from the 2nd Duke’s collection, spanning the 19th century and the opening years of the 20th century, complemented by several pieces which were originally owned by the 1st Duke of Cambridge, his father.

They comprise about 80 lots with a combined pre-sale estimate in the region of £800,000.

James Morton, from Morton & Eden, said: “The collection has remained in the hands of the Duke of Cambridge’s family since his death in 1904 and has for many years been displayed publicly at the Guards Museum, Birdcage Walk.

“Throughout his long life, which spanned that of his cousin and close confidante Queen Victoria, George Duke of Cambridge played significant, active roles in the military, diplomatic and social evolution of the nation.

“The orders and medals offered in this auction present a unique record of service and honour, ranging from an exquisite Garter badge given personally to a young prince in 1835 by his uncle, William IV, to the parure of insignia of the Royal House of Chakri bestowed upon the duke by the King of Siam in 1897.

“The collection is further enhanced by the inclusion of a number of magnificent favourite pieces which originally belonged to the duke’s father Adolphus, the youngest son of George III.”

After a scandal over the duke marrying an actress (it could never happen now…), the dukedom remained vacant following George’s death in 1904 until 2011, when the present queen conferred the title on her grandson, Prince William.