Robbers pulled off what is thought to be Britain's biggest jewellery heist in a 40-million-pound ($68 million) raid at a chic London shop, police say.

Two smartly dressed men armed with handguns walked in to Graff, on London's upmarket New Bond Street last Thursday, and threatened staff before getting away with 43 rings, bracelets, necklaces and watches.

As they left, they dragged a female member of staff with them and fired a warning shot outside the shop, although no-one was injured and the assistant was left behind as they raced off.

Another shot was also fired soon after as the pair abandoned their BMW getaway car nearby and switched to a Mercedes-Benz, before switching to a third car later.

Graff, which is known for its diamonds, was also targeted in 2003 in what until now was reportedly Britain's previous most expensive jewellery robbery, worth 23 million pounds.

Police in London released CCTV images of the suspects on Tuesday and appealed for information about the raid, which took place in broad daylight at 4.40pm (local time).

"These men are extremely dangerous and fired at least two shots in busy London streets as they made their getaway," said Detective Chief Inspector Pam Mace of the Flying Squad, which investigates armed robberies.

"Someone knows who these men are - they would undoubtedly have spoken about it beforehand or boasted about it afterwards."

New Bond Street, in the exclusive Mayfair district, is dotted with big name gem stores including royal jeweller Asprey.

Celebrities spotted wearing Graff's jewels include Paris Hilton, Kylie Minogue, Naomi Campbell, Oprah Winfrey and Victoria Beckham.

- AFP