The world's biggest diamond, believed to be twice the size of the Cullinan, has been discovered in the North-West Province of South Africa, The Guardian reports from Johannesburg.

The Millenium Star (shown above) is the world’s second biggest flawless diamond, weighing 203 carats. The newly-discovered stone could produce an even larger gem.

The find has electrified the diamond community, but the circumstances of the discovery are shrouded in mystery. The diamond is expected to attract furious bidding from buyers worldwide and could fetch up to £15m.

A spokesman for the mining house which made yesterday's find, Brett Joli, said the diamond was being rushed to a bank vault in Johannesburg and would be kept there for a couple of days “until we calm down and decide what we are going to do”.

The South Africa Broadcasting Corporation said the stone was said to be twice the size of the Cullinan diamond.