The 22ct gold, enamel engraved Cadbury's egg was part of the Creme Egg Mystery Conundrum hunt in 1983 and held vital clues for treasure seekers.
It weighs 326.6 grams (11.5 oz) and was designed by the Queen's official jeweller at the time - Garrard & Co.
It is due to go under the hammer on Saturday.
This is the first time it has been on public display.
Cadbury's launched a treasure hunt in 1983 in which 12 gold Conundrum eggs were hidden across the country.
The eggs could be found by solving clues hidden in a book, Conundrum, The Cadbury's Creme Egg Mystery, by Don Shaw.
Hidden eggs were each based on a different story or verse in the book and weighed approximately 250g each.
But most people did not know that a 13th egg was made, bigger than the others with a design encompassing 12 riddles that referred to the locations of the originals.
A spokesman for auctioneers Batemans said: "This egg was the prize in a prize draw Cadburys ran for all of its retailers whilst the Conundrum treasure hunt was going on in 1983.
"Basically, every Cadbury retailer in the UK was entered and our vendors were the lucky winners. This particular draw wasn't open to the public and so this egg, number 13, was never seen by the public.
"It is larger than all the other Conundrum eggs and is based on the cover illustration of the book."
This egg was decorated based on the front cover of the book, incorporating elements of each riddle.
Conundrum is written on top of the egg, along with a tabby cat and weather vane, and around the egg is a star-studded sky and landscape of scenes from the book.
A precious golden egg from a treasure hunt in the 1980s is being auctioned in Lincolnshire with a guide price of between Â£10,000 to Â£15,000.