A dealer bought the toy from a man who had worked in central America
A prototype of a Dinky toy that was never manufactured is expected to fetch up to £10,000, an East Sussex auction house has said.
The light blue 3ins (8cm) Austin A40 van was made for Omnisport, a general store in El Salvador, in the 1950s.
However, an order to make more was never taken up and it remains the only one of its kind in the world.
The toy will be on offer at Wallis and Wallis Auction House in Lewes, East Sussex, on Monday.
Auctioneer Glenn Butler said the van had been brought in by a local dealer who bought it about five years ago from a man who worked in central America in the 1950s.
Mr Butler said that the significance of the toy was not realised straight away, but after consulting with various experts, they were left in no doubt of its value.
He said: "It's created a massive amount of interest. The chances of this happening are incredibly slim, it's like somebody finding a Constable painting that wasn't known to exist before.
"A normal van of this type could reach between £2,000 and £4,000, but having listened to the people who have come in, and knowing how rare the Dinky is, we think it could sell for between £5,000 and £10,000."
Mr Butler said another rare vehicle sold for £12,000 a few years ago.
Dinky toys became popular in the early 1950s, with most of the detailed models having a scale of approximately 1:48.
Although they had no rivals at first, new competition from other toy-making companies meant that they progressed to having working features, such as opening doors and boots.