Metropolitan Police officers who went in search of youths smoking cannabis got more than they bargained for when they discovered a 15-inch knife in Elephant and Castle, south-east London.
The youths ran off when the officers arrived at the scene at 8pm on Saturday night but the deadly weapon had been left behind. No arrests were made.
Sergeant David Hunt posted a picture of the machete-style blade on Twitter with the caption: "Just seized this knife maybe saved another life? Unfortunately didn't catch the youth carrying it."
He added: "The worrying thing is why did they have it in the first place?"
A fellow Met police sergeant, Chris Molnar, wrote: "Great work. I think you've just topped Crocodile Dundee there #ThatsNotAKnife."