Knives have been sold illegally to a 16-year-old in a quarter of shops tested in part of Nottinghamshire.
Six out of 24 shops failed a test purchasing exercise by police and the county council's trading standards in Mansfield and Ashfield last week.
The owners and staff at the shops will now receive a warning from Nottinghamshire Police.
It is illegal to sell knives, with the exception of small, folding penknives, to anyone aged under 18 in the UK.
Shop staff will also be given advice by the county council's trading standards officers on how to prevent illegal sales in the future.
Ch Insp Paul Winter said: "We are working with our partners to reduce the level of violent crime in our area.
"Although we do not have many incidents involving knives, one is too many.
"It's really important for retailers to understand that they have a big role to play in preventing the under 18s having access to knives."