Lancaster police said a knife was sold to an undercover child during a test purchasing operation in the city.
Officers partnered with Trading Standards to test six premises on Wednesday, during Knife Crime Awareness Week.
They said five premises "passed with flying colours" but "one unfortunately failed and sold a knife to our underage volunteer".
"We are following up with enforcement activity to make sure this doesn’t happen again," said a police spokesperson.
"This is just one of many operations we are undertaking this week as a part of Op Sceptre.
"Although this week is Sceptre week of action, we work closely with Trading Standards all year round to regularly check premises' compliance with the law, offer education packages to help prevent sales to young people and then in the unfortunate case of sales being made to someone under the age of 18, enforcement follows."
It is an offence to sell a knife or bladed article to anyone aged under 18, and to be in possession of a knife or any article which has a blade or is sharply pointed, or other offensive weapon in public.
Find out where your local knife amnesty bin is HERE.
Op Sceptre is a week of action against knife crime.
If you have any information about knife crime in your area, call 101 or report anonymously through the independent charity Crimestoppers on 0800 555 111.
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