Under-18s in England are to be banned from buying electronic cigarettes, the government has announced.
Experts say it is not yet known what harm the tobacco-free devices could inflict and that their contents could be damaging young people's health.
An estimated 1.3m people in the UK use e-cigarettes which were designed to help smokers quit.
Ministers also plan to make it illegal for adults to buy traditional cigarettes for anyone under 18.
They want to crack down on the number of young people smoking by bringing the law in line with restrictions on the sale of alcohol.
The new rules could be in force by the autumn and may mean anyone caught buying cigarettes for a child could be given a £50 fixed penalty notice or a fine of up to £2,500.
"We must do all we can to help children lead a healthy life," Public Health Minister Jane Ellison said.
"That's why this measure is designed to help protect children from the dangers of being bought cigarettes by irresponsible adults - something that I hope concerned parents and responsible retailers will welcome."
Some 41% of 15-year-olds who smoke say they usually buy their cigarettes from someone else, rather than from a shop, according to Department of Health figures.
But while smoking rates have fallen to their lowest ever level, experts fear the electronic substitutes could be encouraging teenagers to take up the habit.
E-cigarettes, popular with teenagers, deliver a hit of nicotine and emit water vapour to mimic the feeling and look of smoking.
"We do not yet know the harm that e-cigarettes can cause to adults let alone to children, but we do know they are not risk free," Prof Dame Sally Davies, England's chief medical officer, said.
"E-cigarettes can produce toxic chemicals and the amount of nicotine and other chemical constituents and contaminants, including vaporised flavourings, varies between products meaning they could be extremely damaging to young people's health."
The UK currently has few restrictions on the use of e-cigarettes, despite moves in some countries to ban them.
Restrictions have recently been mooted in Scotland and Wales, where health policy is a devolved issue.
The law change for England will be introduced in Parliament this week as an amendment to the Children and Families Bill.
Anti-smoking charity Ash welcomed the changes but chief executive Deborah Arnott added: "We'd also like to see a retail licensing system put in place so that cigarettes could only legally be sold in shops, not in car boot sales and markets."
Smoking remains one of the biggest causes of death and illness in the UK with around 100,000 people dying each year from illnesses linked to the habit.