A nicotine substitute which can be bought online for 12p more than triples a smoker's chances of quitting for at least a year, research has shown.
Tabex, which contains the active ingredient cytisine, is obtained from laburnum seeds.
Experts believe the drug is as effective as conventional stop-smoking treatments and could save the NHS many millions of pounds a year.
But despite four decades of use in eastern Europe, the pills are unlikely to be available on prescription in the UK for another two to three years.
The British scientist who led the new trial spoke of the "Alice in Wonderland" regulatory system responsible for the delay.
Professor Robert West, from the Cancer Research UK Health Behaviour Research Centre at University College London, said he expected to see a flood of internet orders for Tabex once news about the drug got out.
"It's been available in central and eastern Europe for more than 40 years, we have safety data on millions of people, and we know it's effective, but it's not licensed in Britain," he said.
"People can make their own choices. A licence is not a licence to buy, it's a licence to market. There's nothing illegal about buying this drug online, but there's always the risk that you might not get what you expect."
The Medicines and Healthcare products Regulatory Agency (MHRA), which licences drugs for sale in the UK, warned of the risks of buying unregulated drugs over the internet, however.
A spokesman said: "People are advised that they should take prescription-only medicines after an appropriate consultation with their GP.
"Only healthcare professionals can take into account risks and benefits associated with every medicine.
"Anyone who self medicates and buys their medicines from internet sites could be in danger of receiving counterfeit or substandard medicines. At best these will be a waste of money, at worst they can kill. You don't know what these products contain and you don't know in what conditions they have been made."
The trial, involving 740 patients, showed that people who wanted to stop smoking were 3.4 times more likely to succeed with Tabex than with a "dummy" placebo tablet.
Participants took between two and six pills per day for 25 days. After treatment, 8.4% of those given Tabex were able to avoid smoking for a year compared with 2.4% of the placebo group.
The low overall success rates reflected how hard it was even for motivated smokers to quit, said the researchers.
However Prof West said his team was "extremely encouraged" by the results, published in the New England Journal of Medicine.
"The benefits of Tabex are comparable with those of other smoking cessation treatments, but at a fraction of the cost," he said. "We recognise that stopping smoking can be extremely difficult and we hope that by using cytisine as a substitute for nicotine, the results of this trial could help transform the health of nations around the globe by offering a practical option even for the poorest smokers."
A box of 100 Tabex pills can be bought online for just £12. Prof West said the cost to the NHS would be even cheaper.
He pointed out that the drug works in a similar way to the existing treatment Champix, which costs the NHS £100 to £150 per 12-week course.
"Tabex would cost less than a tenth of that," said Prof West. "If and when it becomes available on the NHS I'd expect to see a reduction of our drugs bill of tens of millions of pounds a year."
The new study, funded by the National Prevention Research Initiative managed by the Medical Research Council, should pave the way to the drug being licensed in the EU and UK.
At least one further trial will be necessary before a licence is granted, said Prof West.
He added: "One of the Alice in Wonderland things about the regulatory system is that it's not designed for drugs that have been on the market for a long time. It's designed for drugs you're bringing from animals to humans."
Jean King, Cancer Research UK's director of tobacco control, said: "It is great news that smokers around the world may have access to a new way to help them beat their addiction. We hope that cytisine will help low and middle-income countries meet their obligations to help treat nicotine dependence under the World Health Organisation's treaty on tobacco."
Doireann Maddock, senior cardiac nurse at the British Heart Foundation, which part-funded the study, said: "We need some bigger trials first, but this pill may yet offer a low-cost treatment to help people break this harmful habit."
Cytisine acts in a similar way as nicotine to stimulate "reward" pathways in the brain.
Prof West sent out a strong warning to anyone tempted to chew highly toxic laburnum seeds.
"Dosing is critical," he said. "That would not be a wise idea. It would probably make you very sick."