West Yorkshire Police are reminding shop-keepers that it is illegal to sell knives to under 18s.
The warning comes after the Force, working with underage cadet volunteers, carried out 23 test purchase operations in small, medium and large sized stores.
Out of the shops tested, seven failed by selling knives to the cadets.
The test purchase operations were part of a national week of action the force supported against knife crime.
They are warning that future test purchase operations will be carried out in the near future and if stores fail the test then enforcement action will be taken against them.

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