The new rule will take effect in England and Wales on October 1, 2007, a government statement said.
Scotland plans to increase the minimum age to 18 sometime in 2007 and Northern Ireland may also raise the age in forthcoming legislation.
The government has already ordered a ban on smoking in all workplaces and enclosed public spaces in England from July 1, 2007.
The government says about nine per cent of Britons between the ages of 11 and 15 smoke, and it is determined to reduce their number.
Raising the tobacco-buying age to 18 will make it easier for retailers to spot under-age smokers and reduce the number of teenagers who become addicted to nicotine, it says.
The change will bring England and Wales into line with several European countries and the United States.
''Smoking is dangerous at any age, but the younger people start, the more likely they are to become life-long smokers and to die early,'' Public Health Minister Caroline Flint said in a statement.
''Buying cigarettes has been too easy for under-16s ... The law change demonstrates our determination to stop this and to reduce the number of teenagers who smoke,'' she said.
The government decided to change the law after consulting the public, retailers, the National Health Service and local authorities, among others.
REUTERS AKJ HS0836