Tills could become "conflict flashpoints" when the legal minimum age for buying cigarettes and knives increases from 16 to 18, according to the British Retail Consortium.
The change comes into force on 1 October, and the BRC has warned that if consumers are not given enough information, there could be an increase in threatening behaviour, verbal abuse and violence against shopworkers.
"Checkouts are conflict flashpoints where restricted goods such as alcohol, cigarettes and knives are concerned," said BRC director general Kevin Hawkins. "Saying 'no' can be unpleasant, particularly in the face of intimidation, verbal abuse and even violence. Retailers are keen to contribute to reducing smoking and knife crime among young people. But they will need the support of the police and communities."
The government needed to ensure teenagers were aware of the changes and the police should be ready for more conflicts, he added.