Police foiled the "robbery of the Millennium" when they caught raiders, including a Brighton man, smashing their way into the Dome with a mechanical earthmover, a court heard.

Martin Heslop, prosecuting at the Old Bailey, said: "It was a remarkable police operation. But for their intervention, they would have got away with diamonds worth £200 million."

He was opening the trial of six men accused of plotting to rob the De Beers Millennium Diamond Exhibition between July 7 and November 8 last year.

Kevin Meredith, 34, of Auckland Drive, Brighton, Wayne Taylor, 35, of Tonbridge, Kent, Aldo Ciarrocchi, 31, of Balaclava Road, Bermondsey, London, William Cockran, 48, of Catford, London, Robert Adams, 57, and Raymond Betson, 39, both of no fixed address, all deny conspiracy to rob.

Mr Heslop said the gang's target had been "nothing less than the 12 extremely rare and valuable diamonds" on display at the Dome.

He said the Millennium Star and the Millennium Blue Diamonds "may very well be the rarest and finest diamonds in the world".

Mr Heslop said: "They were playing for very high stakes. This was no ordinary robbery. The value of the diamonds is conservatively estimated, and I will pause here, at £200 million.

"Had they succeeded, it would have ranked as the biggest robbery in the world in terms of value. It could properly be described as the robbery of the Millennium.

"It was planned professionally and carefully down to the last detail. The conspiracy was so well organised, it almost succeeded. They smashed their way into the Dome at about 9.30am."

Mr Heslop said they caused terror as they sped towards the vault in the JCB and had a boat waiting to take them across the Thames to a getaway van.

He said the raiders inside were equipped with gas masks, smoke grenades and bottles of ammonia to discharge.

They had a sledge hammer, wire cutters and body armour and had spent months on surveillance, including videoing areas.

They acquired premises in the south of England, "away from prying eyes" and highly sophisticated radio and scanning equipment, the prosecution alleged.

Mr Heslop said Meredith was the speedboat driver and was caught on the boat.

The trial continues.