A new law making it illegal for under-18s to buy tobacco comes into force.
The legal age for buying tobacco rises from 16 to 18, bringing it into line with alcohol.
Health campaigners welcomed the move, saying it would be easier for retailers and customers to follow the law.
But critics accused the Government of a "woefully inadequate" advertising campaign, saying only one in five 16 to 18-year-olds knew about the change.
The Department of Health hopes the move will cut the number of teenage smokers and has promised a hard crackdown on retailers who flout the law.
Shop owners will have to display signs in their windows and could face fines or prosecution if they sell tobacco to those under-age.
A spokesman for the British Retail Consortium said it supported a move to bring tobacco in line with alcohol, fireworks and glue.
"Retailers are well aware of the change and have been doing their bit to inform customers," he said. "But, from today, we will see how successful the Government's promised campaign has been in preventing stores becoming conflict flashpoints as shop staff say "no" to 16 and 17-year-olds who have been able to buy cigarettes previously."
The Association of Convenience Stores, which represents around 33,000 local shops, criticised the Government's "largely internet-based" advertising campaign for shoppers.
Its chief executive James Lowman said: "The Government's communication campaign has been woefully inadequate. The risk that low awareness presents is that people will blame the retailer for refusing to sell them a product they were previously able to buy. This will lead to flash points, abuse, intimidation and even violence."
