A diamond thought to be the fifth largest ever found has been discovered in a mine in the southern African country of Lesotho.

The diamond is estimated to be worth at much as $40 million (£30million), according to experts.

The 910-carat stone was unearthed at the Letseng mine in Lesotho by mining company Gem Diamonds.

A diamond thought to be the fifth largest of gem quality ever found has been discovered in Lesotho, miner Gem Diamonds said.

The company described the diamond, which is the size of two golf balls, as being of 'exceptional quality'.

'Since Gem Diamonds acquired Letseng in 2006, the mine has produced some of the world's most remarkable diamonds, including the 603 carat Lesotho Promise,' Gem Diamonds chief executive Clifford Elphick said in a statement.

'However, this exceptional top quality diamond is the largest to be mined to date... This is a landmark discovery.'

Ben Davis, a mining analyst at Liberum Capital, speculated in a research note to investors that the diamond could be worth as much as $40 million.

Gem Diamonds shares in London were up 14 percent from the market open to £0.92 a piece.