Officers carried out 38 alcohol and 30 tobacco test purchases to target “likely problem premises" in Crook, Stanley, Barnard Castle, Ferryhill and Consett.
Six alcohol and five cigarette sales were made to under-age volunteers resulting in fixed penalty notices, written warnings and potential prosecutions.
Durham County Council said the intelligence-led operation was designed to raise awareness and provide advice for businesses as well as identify those breaking the law.
Those who’ve failed test purchases have been encouraged to adopt a Challenge 25 scheme
Joanne Waller, head of environment, health and consumer protection, said: “It’s clear that staff still do not appreciate how difficult it is to judge the age of teenagers and so should be rigorous in their application of the Challenge 25 rule and always ask for ID.
“Follow-up visits will be made to all those who made illegal sales and, if there are more, prosecution or an application to review a licence will be the likely outcome.”