Almost one in five retailers illegally sold cigarettes to children during a test purchase operation coordinated by the Chartered Trading Standards Institute (CTSI).
While the majority of retailers complied with the law, smaller grocery stores, newsagents and petrol stations were found to be most likely to sell tobacco products to under 18s.
The targeted-approach tested retailers across England including those in the south east, south west, central England, north west, east of England, north east, east Midlands, London and Yorkshire and the Humber.
Results from the investigation form part of a report - published today - which looks at the harm caused by underage tobacco sales and the work to prevent it.
Leon Livermore, CTSI chief executive, said: “More than 200,000 children aged 11 to 15 start smoking each year and evidence shows these younger smokers are more likely to become heavy smokers with higher levels of dependency and mortality.
“Trading standards teams play a frontline role in preventing children from obtaining cigarettes and developing a habit that is detrimental to their health and a costly burden to the health service.
“Local trading standards services can and often do prosecute offending retailers but they also issue warnings and provide guidance to retailers, as part of the national drive to stamp out smoking."
CTSI’s Enhancing Local Tobacco Control report was commissioned by the Department of Health’s tobacco policy team and includes details of 352 individual test purchases across the nine regions.
In each case young people were supervised by trading standards officers during covert operations as they attempted to buy tobacco products, between December 2014 and March 2015.
Sixty-three illegal sales were made with the highest percentage of underage sales made from small retailers (24%) independent newsagents (16%) and petrol stations (20%).
According to the operation, London performed poorest with almost one in four (24%) illegal sales whereas about one in ten retailers the south east (9%) and south west (12%) made illegal sales.
Jane Ellison, public health minister, said: “Two-thirds of smokers start smoking regularly before the age of 18 so stopping children from buying cigarettes is vital and we need the support of local authorities and retailers to do this.
“We are responding to calls from retailers to help them by changing the law to make it illegal for someone to purchase cigarettes for anyone under 18 from October.
“This report shows how local authorities can target their enforcement work. Our aspiration is for a smoke-free generation and we want to make sure we are all doing all that we can to protect children from tobacco - the biggest preventable killer in this country.”
The overall 18% failure rate is higher than that of recent National Tobacco Control Surveys which returned 10% in 2013/14 and 15% in 2012/13. The CTSI results will form part of the next survey.
A report published by Cancer Research UK estimates that 207,000 children aged 11 to 15 start smoking annually and two-thirds of adult smokers became addicted as children.
In 2007 the legal age for tobacco sales was raised from 16 to 18 as part of a range of measures to discourage the habit, including a ban on smoking in enclosed public spaces.