Owned for many generations by the Kington family, the elaborately decorated emerald green quilted saddlecloth will go under the hammer at Dreweatts' sale of militaria in Donnington Priory near Newbury on September 26.
Malcolm Claridge, militaria specialist at the auction house, said: "It is an absolutely unique Royal artefact from the Elizabethan era and is in extremely good condition considering it is 438 years old."
According to the family, the piece has never been offered at auction before.
Until recently it was in the possession of the late writer Miles Kington, who joked that it was cursed in a lengthy fax to his wife Caroline shortly before his death, a copy of which is included in the lot.
In it he says: "But it's just a silly old priceless family heirloom sitting in an old glass case, I hear you laugh. There's nothing secret about it at all.... Ah, would that it were so."
Mounted for display in a 4ft 1in (1.25m) square glazed case with a plaque, the piece is estimated at £8000-10,000.
Tel: 01635 553553.
