The government announced this week that it was considering increasing the minimum purchase age for cigarettes from 16 to 17 or 18 and imposing tougher sanctions on retailers that sell to under-age teenagers.
The ACS said that it backed tough penalties for retailers flouting the rules, but warned that a change could create problems for responsible c-store operators faced with youngsters who did not know that the law had changed.
Shane Brennan, ACS government relations manager, said: "We are concerned about how a change in the law may be brought in. It needs to be carefully planned, as there is potential for retailers to be exposed to being the enforcers of the change.
"There needs to be clear communication on the change, not only to retailers but to the public as well.
"Research by Usdaw indicates that the factor that causes abuse or violence the most is refusal of underage sales, and we really have to be fully prepared for that."
The government said that it was reviewing the penalties for retailers selling cigarettes to children.
It claimed that figures showed that 70% of 11 to 15-year-old smokers had bought their tobacco from small local shops.
Among the punishments under consideration is the withdrawal of an offending retailer's right to sell tobacco.
Caroline Flint, minister of state for public health, said: "Smoking is dangerous at any age, but the younger people start, the more likely they are to become life-long smokers. Access to cigarettes by under-16s is not as difficult as it should be and this is partly due to retailers selling tobacco to those under the legal age.
"If a particular shop is known locally as the place where children and teenagers can buy tobacco easily, we want to stop that shop selling it."
Brennan said: "The ACS supports tough sanctions against retailers that persistently sell tobacco to the underage, but we are concerned that the existing penalty regime is not being used effectively."