Officers found alcohol being sold to underage drinkers in four out of six shops targeted during an undercover operation.
Police and Trading Standards ran a test purchase operation at off-licences in Wellingborough, Irchester and Finedon on Saturday, August 31.
PC Paul Hollands said: “We had information that these particular premises may have been selling alcohol to underage customers and four out of the six that we went into as part of the operation did.
“Selling alcohol to children is illegal and we use all the powers that we can to take action to stop this.
“The vast majority of off-licences run their businesses legally but this is a clear message to the minority that don’t.
“We regularly receive information from people who are concerned that their local shop is selling to children and we act accordingly.”