Miners in the southern African kingdom of Lesotho have found one of the world's largest diamonds, a near-flawless white gem weighing nearly 500 carats, mining group Gem Diamonds said on Sunday.

The diamond was discovered in the Letseng Mine on September 8, the company said in a statement. It has been analysed by experts in Antwerp and found to weigh 478 carats, with very few inclusions and of outstanding colour and clarity.

"It has the potential to yield one of the largest flawless D colour round polished diamonds in history," the company said.

The world's largest diamond is the Cullinan, discovered in South Africa in 1905. It weighed more than 3,100 carats before it was cut into more than one hundred separate gems, many of which are part of the British crown jewels.