Teenagers are to be used to snare rogue shopkeepers who sell knives to under-18s.
They will play a major part in a blitz on stores which help fuel Britain's growing blade epidemic.
It is feared many traders are going unpunished despite laws brought in last year banning the sale of knives to under-18s.
Now the Home Office has ordered police and Trading Standards to come down hard on shops in the 10 worst knife crime
The Met's deputy assistant commissioner said: "This cannot be down to police alone - retailers must take responsibility for their actions."
Geoffrey Theobald, head of council trading standards organisation Lacors, revealed previous operations using young volunteers had shown most stores obey the law.
He added: "There's still a minority prepared to sell knives to children.
"Councils aren't trying to hoodwink retailers and will only use volunteers who look under 18."