Gem raiders' haul valued at £380,000

The thieves snatched the haul from the De Beers store in the Royal Exchange shopping arcade in the City of London.

One source said the gang also grabbed some watches from a branch of Omega almost as an after-thought in the five-minute raid.

It is understood the shopping courtyard was protected only by wrought iron gates joined by a chain and padlock more suitable for securing bicycles.

Once inside, the gang used sledgehammers to smash down two sets of glass doors before battering their way inside the De Beers store adjacent to the exit.

The shopping arcade, based around a champagne and seafood bar, is lined by jewellers and luxury goods stores which often leave displays fully stocked to encourage window shopping.

The thieves struck at 10pm on Saturday when a red BMW pulled up outside the listed Royal Exchange, just yards from the Bank of England.

Police believe they used boltcutters to remove the chain and padlock securing the iron gates at one of the four entrances before attacking the glass doors.

Investigators are examining closed circuit TV images from the arcade and surrounding streets, as well as number plate recognition cameras, in a bid to identify the car.