The auctioneer Bonhams says the carved granite step is a Sandakada Pahana - also known as Moonstone - similar to those found in temples dating from Sri Lanka's Anuradhapura period (c400BC-1000AD).
The magnificent work of art featuring lions, horses, elephants, birds and Brahim cows, has come to light in the garden of a Devon bungalow.
It is estimated to attract bids in excess of 30,000 pounds.
The beautiful 1,000 year old pre-Hindu stone step is one of only six examples known to date from the period, making this discovery the seventh.
The temple step is a feature unique to Sinhalese architecture in Sri Lanka.
The massively heavy - three-quarters of a tonne stone - measure eight foot by four foot and is six inches thick.
Sam Tuke of Bonhams in Exeter says of the discovery: "I met the client when she was collecting an item from our office. She mentioned in passing that she had a large slab of carved granite that had come from her mothers house in Sussex and that she had known and loved it since she was four years old.
She loved running her fingers around the animals carved into the stone."
"I said that it sounded an interesting object and she arranged to drop a photograph into me the next day. When I saw the photographs and she explained the full story, I knew that it could be of great historical interest and importance.
The house in Sussex had been bought from a tea planter in the 1950s and the stone had been moved six times.
Her brother had seen similar stones in Sri Lanka whilst on holiday. She explained that she could not bear to leave the stone behind after her father died and the house was sold.
It has been known affectionately in the family as 'The Pebble' and is currently lying outside the front of their bungalow at the end of a concrete path."
The beautifully carved stone features a curved procession of animals including lions, horses, elephants, birds and Brahim cows.
Anuradhapura, the greatest monastic city of the ancient world, was the cradle of the glorious Sinhalese Buddhist civilization.
Today Anuradhapura, a UNESCO World Heritage Site, is replete with renovated monuments, restored edifices, preserved ruins and historical sites where the archaeological excavations are still being continued.