A Richmondshire store was one of 16 across the county which sold cigarettes to a 15-year-old in a recent test purchase survey by North Yorkshire County Council Trading Standards.
A criminal offence is committed if age-restricted products are sold to minors.
The council says that officers will work with participating outlets, who have not been identified, to introduce an effective age verification policy and to help to train staff.
Trading Standards undertake regular test purchasing and a summer survey has found that at 34 per cent of the retailers tested, 15-year-olds were able to buy cigarettes.
The volunteer child helpers who assist Trading Standards have a strict rules to follow in that they are instructed to tell the truth at all times and to provide identification showing their true age if requested.
Councillor Andrew Lee, the council’s executive member for growth, planning and trading standards, said: “This is a reminder of the significant role that local businesses and retailers play in protecting our children’s health by preventing them from accessing all forms of tobacco and nicotine.
“Cigarettes are age-restricted to protect the health of our young people and retailers are urged to be aware of the implications of underage sales.’’
Trading Standards is inviting retailers to work with them to achieve a new Responsible Retailer award, which recognises those who follow under age sales legislation.