Secret underage shoppers have been visiting off-licences in Huddersfield as part of an undercover police operation.
The young people have been deployed by West Yorkshire Police in an effort to test whether shops would sell alcohol to underage customers.
A total of seven different premises were visited by the young shoppers, and police said one of those sold alcohol to someone under 18.
The shop worker who made the sale is being prosecuted.
Across Kirklees, most licensed premises are asked to operate Challenge 21 - a scheme where anyone who appears to be under the age of 21 is asked for proof of age.
PC Leon Stansfield, Kirklees licensing officer, said: "Test purchasing will continue throughout Kirklees throughout the Christmas and New Year, so I would like to remind all licensed premises of their responsibility to promote the four licensing objectives and operate Challenge 21.
"If anyone needs to report a licensed premises for selling alcohol to persons under the age of 18, please contact Kirklees District Police licensing via 101."