It can penetrate up to 25 millimetres or one inch of solid steel

Martin Heslop QC Prosecution

Taking the unloaded gunpowder-powered gun in his hands and pulling the trigger, Mr Heslop told the jury: "You hold it to the surface of the object you want to penetrate.

"It is designed to fire into concrete and steel and can penetrate up to 25 millimetres or one inch of solid steel."

The armoured glass in the Dome vault was three-quarters of an inch thick.

Mr Heslop said the jury would later be told the gun was the one allegedly used by another of the gang - William Cockran - to fire nails into the vault on 7 November last year.

Charges denied

Police foiled the planned raid after mounting a sophisticated surveillance operation involving undercover officers and CCTV cameras, Mr Heslop earlier told the court.

In the dock are Kevin Meredith, 34, from Brighton, East Sussex; Wayne Taylor, 35, of Tonbridge, Kent; Aldo Ciarrocchi, 31, from Bermondsey, and Mr Cockran, 48, of Catford, both south-east London; Robert Adams, 57, and Raymond Betson, 39, of no fixed address.

They all deny conspiracy to rob.

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