James Rowley, 34, was at a Lloyd’s bank in Coulsdon, Surrey, when criminal, Andrew Fernandez, ran in waving a gun and demanded money.
Incredibly, the father-of-two, “called his bluff”, correctly guessing the hand gun was fake, causing Fernandez to hesitate, before fleeing.
He then gave chase, with a colleague who was waiting outside in his van, and detained him until police arrived.
Speaking about the incident to the London Evening Standard, James said: “He got me on the wrong day.
“I didn’t know for certain that it was an imitation firearm, but I called his bluff. I said a few words to him and he hesitated, that’s when it clicked it was a fake.
“I’ve never been in a situation like that before, I just acted on instinct.”
James will be awarded a Police Public Bravery Award in a ceremony on Thursday.
Detective Sergeant David Barnfather, from the Met’s Flying Squad, praised his courage, saying:
“The bravery of James Rowley was truly remarkable in confronting what was for all intents and purposes, an armed and extremely dangerous criminal.
“His courage and sense of civic duty are to be applauded and thanks to him Fernandez is behind bars.”