Undercover cadets helped police target stores that sell knives to young people in Barnet.
Members of Barnet Volunteer Cadets, all aged 15, visited 17 premises across the borough and were able to buy knives in three of them.
Police will now take action against the proprietors.
PC Sandy Clifford said: “The work of the volunteer police cadets is vital in improving links between youth and the police and protecting the public.”
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