The museum took the dramatic step of shutting down its entire Diamonds exhibition three months early after receiving a warning from the police of a planned violent raid. Its staff were at risk, it was told by the police, in no uncertain terms.

Detectives feared that spectacular rare white and coloured diamonds, including the De Beers Millennium Star and the Steinmetz Pink, were about to be stolen. The Millennium Star, at 203.04 carats, is one of the largest flawless diamonds in the world. It has been targeted before, in the bungled Millennium Dome raid.

The exhibition — arguably the greatest assemblage of diamonds staged at the same time — was closed “permanently and with immediate effect”, the museum said yesterday.

Michael Dixon, its director, said: “Since we began planning this exhibition, we have followed police advice to the letter in terms of ensuring the security of our staff, our visitors and the exhibition specimens.

“That advice changed on Tuesday afternoon. It indicated a heightened criminal risk to the exhibition. The museum’s priority is the safety and security of our visitors and staff.

“Based on police advice, the only responsible course of action in this situation was to close the exhibition.”

A spokeswoman for Scotland Yard said: “We have been working with the Natural History Museum and decided to close down the exhibition.

“Information we have received leads us to believe that criminals were looking to stage an attempted robbery on an exhibition in the museum.”

The diamonds will now be returned to owners worldwide.

At 59.60 carats, the Steinmetz Pink is one of the world’s finest pink diamonds and was first revealed in 2003. It took almost two years to cut. Pink diamonds are extremely rare and usually far smaller in size.

Other exhibits that may well have caught the eye of the robbers were Mogul treasures, including the Shah Jahan table-cut diamond, the George III Garter Star and Queen Victoria’s Lesser George, on loan from the Royal Collection.

The Eureka diamond is significant because it was cut from the first authenticated diamond found in South Africa, in 1866. The four men who planned the £200 million diamond robbery at the Millennium Dome in 2000 were caught after they had broken through the perimeter fence. They had been given inside help by members of staff.

Charlie Hill, the former head of Scotland Yard’s Art and Antiques Squad, said that the police were able to foil that robbery because they had good informants. He added: “But you can’t sit in a museum for ever guarding the exhibition.

They’ve taken the most costeffective measure in closing it down.”

The museum’s director said: “We are very disappointed. The security risks have been reassessed on an almost continuous basis. The perceived level of risks changed yesterday.”

The exhibition brought together many of the most impressive single stones in the world and showed that not all diamonds are “white”. For every 10,000 white diamonds, one coloured one is mined.

The robbers must be among more than 70,000 people who have visited the exhibition since it opened 4½ months ago.

THE DE BEERS MILLENNIUM STAR

At 203.04 carats, one of the largest flawless diamonds in the world. Internally and externally flawless. Unveiled in 1999 at the Millennium Dome, it became the target of a foiled robbery attempt in 2000. This is the first time since then that it has been on public display in Britain

THE STEINMETZ PINK

The world’s largest fancy vivid pink, flawless diamond, loaned by Steinmetz, which took almost two years to cut this magnificent 59.60-carat oval-shaped diamond. First unveiled around the neck of model Helena Christensen in Monaco, May 2003

THE MOUSSAIEFF RED

There are very few true red diamonds in existence. To find a deep red diamond of this size — 5.11 carats — is astounding, making it one of the most rare and highly prized diamonds in the world

THE BLUE EMPRESS

This 14-carat pear-shaped blue diamond was one of 11 fancy blue diamonds, which formed the unique Midnight Collection, shown by De Beers in 1999. Extremely rare

THE OCEAN DREAM

The world’s largest naturally occurring fancy deep blue-green diamond at 5.51 carats. Its colour is caused by exposure to natural radiation over millions of years

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