The man was arrested at an address in Illford, east London, on Monday, four days after the robbery, which took place at the Graff Diamond jewellers on New Bond Street at 4.40pm on Thursday.
The man, who has since been released on bail, is not one of the two men in the CCTV footage released by the Metropolitan Police. A police source described him as a "minor player".
This morning mobile phone footage emerged which showed the moment that two men escaped with their haul. The video was taken by a man who just happened to be filming on New Bond Street at the time the robbery happened.
In the video the two suited suspects are clearly seen leaving the store. A man's voice shouts "call the police" before a gunshot is fired and screams are heard.
The footage then distorts and re-starts with a woman talking. a man reassures her: "They are gone, they are gone. Calm down, calm down."
The men stole 43 pieces of jewellery in two minutes before taking a female staff member hostage to make their escape.
Their haul included a pair of white, round, diamond double-hoop earrings, a yellow diamond flower necklace, a platinum white Marquise diamond ring and a Chronograph men's 45mm watch.
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