Gems worth up to £3 million were deftly stolen from the exclusive Masterpiece art fair in Chelsea, west London, that positively swarms with round-the-clock security.

With no apparent witnesses, there was little to explain how or when the thieves targeted the stand belonging to Swiss jeweller Boghossian, leaving insiders forced to speculate.

One theory is that the crime involved professional thieves who used distraction techniques.

A source at the fair said the exhibition, which features sculpture, haute Parisian décor, fine jewellery or contemporary art, had been protected by security guards and that organisers were working with police.

Workers said they were “amazed” by the audacity of the theft.

One told the Evening Standard: “There were security (staff) everywhere. It was not the normal hospital security — these were all suited and booted and really meant business.

“I am very surprised this has happened. It’s like the Pink Panther films or something.”