Nearly ten percent of Britain's 11 to 15-year-olds smoke and the government hopes that raising the minimum age will not only reduce their access to cigarettes but also help retailers more easily identify under-age smokers.
The government also hopes that bringing the laws on the purchase of tobacco into line with those on buying alcohol will highlight the health risks associated with cigarettes.
Earlier this year the government launched a website and advertising campaign aimed at informing teenagers of the change in the law, after research showed that only 25 percent of children aged under 16 were asked to prove their age when they tried to buy tobacco.
Retailers will now risk prosecution and a fine of up to 2,500 pounds (3,590 euros, 5,100 dollars) if they sell tobacco to anyone aged under 18.
Anti-smoking campaign group ASH welcomed the decision to raise the legal age and urged that it is properly enforced.
"It is imperative that retailers fully comply with the law and that enforcement officers make frequent checks on premises selling tobacco," ASH spokeswoman Amanda Sandford told the BBC.
Pro-smoking lobby group FOREST criticised the measure. "You're considered old enough to have sex at 16, drive a car and join the army at 17, but the government doesn't want you to smoke until you're 18," spokesman Simon Clark said.
The Department of Health said the new legislation was part of the government?s attempt to reduce the number of smokers in Britain.
"The age of sale legislation shows our commitment to continue to drive down smoking rates in the UK as smoking remains the number one cause of ill health and early death," a spokeswoman said Monday.
The new measure follows on from smoking bans across Britain which have made it illegal to smoke in public places.