A Doncaster shop sold a knife to a child during an undercover operation led by police.
Officers in plain clothes along with a youngster under the age of 18 tested 17 stores on the town centre last Friday with one failing.
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PC Jack Goodall, of the Doncaster Central Neighbourhood Policing Team, said: "The store that failed will be placed on an action plan and be supported by South Yorkshire Police to prevent this from happening again.
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"The effects of knife crime can be devastating.
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"So if you know anyone who works in retail, please remind them that it is an offence to sell knifes to a person under the age of eighteen.
"And that it carries a maximum of imprisonment term of six months and/ or a fine."