An investigation has found almost a fifth of retailers sell cigarettes to children with the figure rising to around a quarter in London.

Petrol stations, small grocery stores and newsagents are the worst offenders, according to the Chartered Trading Standards Institute (CTSI).

Its operation carried out more than 350 test purchases across England’s nine regions. Of the illegal sales, 24% were in London compared with just 9% in the south east.

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.’

The CTSI’s investigation forms part of a report commissioned by the Department of Health’s tobacco policy team. In 2007 the age for tobacco sales was raised from 16 to 18 and from October this year it will be illegal for anyone under 18 to purchase cigarettes.

Jane Ellison, public health minister, said: ‘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.’