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Research shows that tobacco companies have increased the nicotine yield in cigarettes by 11 per cent over the past seven years, and that that nicotine yield...

A major U.S. study has shown that nicotine levels in cigarettes have been increasing steadily for years.

Harvard researchers say this is not an accident, and that cigarette manufacturers are trying to make it more difficult for smokers to quit.

Research shows that tobacco companies have increased the nicotine yield in cigarettes by 11 per cent over the past seven years, and that that nicotine yield per each smoked cigarette has risen 1.6 per cent annually between 1997 and 2005.

Researchers claim the findings could mean that cigarette companies are violating terms of the 1998 U.S. nationwide tobacco settlement, which requires the industry to work to prevent underage smoking.

American cigarette companies disputed the findings when they were initially released last year, and argued that the variations in nicotine yield are random.

The new Harvard study was introduced, in part, as an independent confirmation of the results.