The Office of Tobacco Control (OTC) said the number of 15 to 18 year olds who believe they can buy a pack of 20 has fallen from a third since an in-store advertising ban came into effect a year ago.

Norma Cronin, OTC chairperson, praised the ban saying it would help to reduce the number of young people taking up smoking in the long term.

"Once again, Ireland has displayed leadership in tobacco control as we continue to restrict opportunities to promote tobacco to the next generation," she said.

However, Ms Cronin added: "The fact remains that one third of retailers are still prepared to sell tobacco to minors and this is not acceptable.

"I urge all retailers of tobacco to ensure that tobacco is not sold to minors and thus prevent the start of a serious addiction to nicotine."

Since July last year, shopkeepers have been barred from using promotional, marketing or advertising material to display cigarettes for sale in-store.

A survey of teenagers commissioned by the office during 2009 showed support for the ban grew from 56% before it was introduced to 68% afterwards.

It also showed 33% of those surveyed thought they or their friends could successfully buy cigarettes before the ban while afterwards only 25% thought they could get away with it.

The OTC said the percentage of retailers refusing to sell cigarettes to under 18s had increased to 68%, compared to a 60% in 2008 and 52% in 2007.