Almost a third of shops tested in a county-wide survey would sell cigarettes to an under-age child.
Eight of the 29 shops visited in East Sussex failed to ask for proof of age from a 14-year-old volunteer, sent in by Trading Standards officers working for East Sussex County Council.
Each shop has been investigated and warning letters and formal letters handed out. In one case, criminal proceedings have also started.
Those retailers who refused to sell tobacco and asked for identification were sent a letter congratulating them.
Councillor Meg Stroude, lead Cabinet member for legal and community services, said: "Controls on sales to young people are there for good reason.
"It is all too easy to be lured into smoking when you are young, ignoring the health consequences in later life.
"We take very seriously our duty to protect young people and we are encouraging retailers to ask all youngsters for proof of age, not just those who obviously look under-age."
Throughout the rest of the year teenage volunteers will also be trying to buy cigarettes from vending machines, alcohol, fireworks and videos.
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