More than a third of businesses tested in a Leicestershire trading standards' probe were caught selling cigarettes to children.
Over the past few weekends, the trading standards team worked with children to carry out test purchases in 28 shops.
Ten shops failed the test operation and were caught selling tobacco products to under-18s.
Any traders taken to court over illegal sales could be fined up to £2,500, Leicestershire County Council said.
David Bull, head of the authority's trading standards, said: "We're disappointed that this operation found 10 shopkeepers breaking the law.
"We're going to make similar test purchases over the next few weeks so I'd remind businesses that they should ask for photographic identification that a person is 18 or older... before selling tobacco products to young people."