The finding, announced on Wednesday, is likely to fuel support for more regulation as the United States Congress considers legislation that would give the Food and Drug Administration (FDA) broad authority to regulate tobacco.

'Menthol stimulates the cooling receptors in the lung and oral pharynx,' said Dr Gregory Connolly of the Harvard School of Public Health. 'It makes smoking easier.'

Menthol cigarette brands have been rising in popularity with adolescents and this highest use has been among younger, newer smokers.

A 2006 survey in the US found that 44 per cent of smokers aged 12 to 17 reported using menthol cigarettes, as did 36 per cent of smokers aged 18 to 24.

Dr Connolly and colleagues studied internal company research on menthol use released as part of a large tobacco settlement.

They also conducted independent laboratory tests and reviewed population studies on smoking trends.

According to the study, in 1987, R.J. Reynolds - which makes the Camel and Kool cigarette brands - identified low- menthol varieties as a new strategy to recruit new, young smokers.

'First-time smoker reaction is generally negative,' it said in a company document. 'Initial negatives can be alleviated with a low level of menthol.'

And while Dr Connolly said it was clear that tobacco companies use mild menthol cigarettes to target younger smokers, he also said that stronger menthol flavours were targeted at established smokers.

While saying that 'menthol helps the nicotine go down', he also explained that too much menthol is hard for new smokers to tolerate, although hardened smokers prefer a higher level.

Among the brands tested, the researchers found those with the greatest market growth among young adults - Lorillard's Newport brand and Philip Morris USA's Marlboro Milds - had the lowest menthol levels.

Representatives of all three tobacco companies have dismissed the claims that they had engineered their cigarettes to hook youngsters and maintain adult addiction.

The Bill before the House of Representatives prohibits fruit- and candy-flavoured cigarettes but exempts menthol from an immediate ban, although Dr Connolly said it would grant the FDA the power to regulate menthol levels.

REUTERS, NEW YORK TIMES