A large diamond ring bought for just 10 pounds at a car boot sale 30 years ago is expected to fetch a whopping 350,000 pounds at an auction in the UK.

The owner believed the large stone was a piece of fake costume jewellery when she bought it at West Middlesex Hospital in west London in the 1980s and would wear it daily.

However, on evaluation it turned out to be a 26-carat, cushion-shaped white diamond cut in the 19th century.

“The owner would wear it out shopping, wear it day-to- day. It's a good looking ring. But it was bought as a costume jewel. No-one had any idea it had any intrinsic value at all. They enjoyed it all this time,” said Jessica Wyndham, the head of the jewellery department at Sotheby's auction house in London.

“They'd been to quite a few car-boot sales over the years. But they don't have any history of collecting antiques and they don't have any history of collecting diamonds. This is a one-off windfall, an amazing find,” she said.

The owners, who have chosen to remain anonymous, bought it into Sotheby's after a jeweller pushed them to get it evaluated as it could have substantial value.

The diamond was cut in the 19th century but its history and how it arrived at the car boot sale are unknown.

“With an old style of cutting, an antique cushion shape, the light doesn't reflect back as much as it would from a modern stone cutting… it could trick people into thinking it's not a genuine stone,” explained Wyndham.

The diamond ring will be offered as part of Sotheby London's Fine Jewels sale on July 7.