The robbery took place shortly before 12.45pm on Friday, 19 July, at the Western Union branch in Finchley Road, NW3.
Two men entered the store - one brandishing a handgun, the other armed with a sledgehammer.
The suspect holding the gun threatened to shoot staff and demanded they hand over money.
Meanwhile, the second suspect repeatedly struck the glass screen that separates staff from customers with the sledgehammer.
Between £8,000 and £10,000 was given to the men by a terrified staff member.
They then ran south down Finchley Road before turning left into Fitzjohns Avenue. They then got into a waiting car, a silver Renault Laguna, and drove away north, in the direction of Hampstead.
More top news
The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Sam Allardyce has quit as manager of Crystal Palace and retired from football management.
Authorities named Salman Abedi as the suspected attacker who detonated a bomb at the Manchester Arena.