The stone - worth £400,000 - was part of a window display at the Graff jewellery shop in Knightsbridge.

The two robbers, armed with a revolver and a hand grenade, got away in less than a minute with a total of £1,429,000 worth of diamonds.

No one was hurt during the raid, and customers on the other two floors of the three-storey shop were unaware of anything taking place.

Hand grenade

A security guard let the first well-dressed thief into the exclusive store, thinking he was a customer.

Once inside the man - dressed in blue check trousers, jacket and a trilby - pulled out a gun and ordered the staff and customers to lie down on the floor.

The second robber then walked in brandishing a hand grenade.

The Marlborough diamond, another jewel worth £200,000 and 18 other stones were packed into a briefcase by the robbers before they fled to a getaway car parked about 50 yards (45.7 m) away.

Despite being threatened with the hand grenade, one of the assistants at the shop followed the men and noted the registration number of the Fiat Mirafiore they used to escape.

The owner of the shop, Laurence Graff, said the thieves knew exactly what they wanted.

"They picked out the very special pieces from the window... It was all over in less than a minute," he said.

But Mr Graff added they would have great difficulty selling on the Marlborough diamond as it had been re-cut in a very distinctive modern style.

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