'He was dressed extremely smartly in a suit and hat, just like Humphrey Bogart,' said a police spokesman. 'He was in his 50s, and looked pretty distinguished, with a cultured voice.'
Saturday's raid on Chopard - a favourite of Hollywood stars including Kate Winslet and Penelope Cruz - was the latest in a long line of audacious jewellery robberies.
In December an armed gang dressed as pantomime dames got away with £80million worth of jewels from the nearby Harry Winston store.
The culprits have never been caught, but suspicion fell on a gang known as the Pink Panthers, who have stolen up to £200million worth of goods in raids on some 120 jewellers around the world since their first in Mayfair in 2003.
On May 13 two alleged members of the gang were arrested in Paris on suspicion of carrying out armed smash-and-grab raids on stores in Monaco, Switzerland and Germany.
Last Thursday Dusko Martinovic - a Montenegrin former soldier who is believed to be a member of the gang - was sentenced to 15 years in jail for a Saint Tropez heist.
French police have described the group's crimes as 'lightning fast hold-ups: daring, but carefully planned down to smallest detail'.
Humphrey Bogart, star of the classic film Casablanca, wore a trademark fedora hat along with a bespoke suit in most of his films.
Jewellery made by Chopard, a Swiss firm, is worn by stars at high-profile events like the Oscars and Cannes film festival.