The resourceful moles of the US anti-smoking movement have unearthed thousands more pages of confidential tobacco company documents that show previously undisclosed research to adjust nicotine levels in test

cigarettes. The documents seem bound to accelerate moves by the US government to declare cigarettes a drug, and to try to regulate them.

This time the company involved is Philip Morris, the world’s largest tobacco enterprise. Last year, documents on nicotine research were circulated from the British-owned company Brown & Williamson, the third-largest tobacco compan

y in the US. The companies have always asserted that cigarettes contain nicotine because it occurs naturally in tobacco; they say they do not manipulate nicotine levels.

Shares in Philip Morris fell sharply after Charles Wall, a company lawyer, acknowledged to the New York Times that the company had carried out extensive research over many years and manipulated nicotine test in cigarettes. But he said the research was never used in creating products for the market.

The new documents, copies of which have been obtained by the Independent, relate to 1970s research projects in the Philip Morris laboratories in Virginia. They show the company studied different levels of nicotine in cigarettes to see what level would be most acceptable to smokers. A 1971 document on a study of smoker acceptability noted: “Our plans are to concentrate upon that nicotine delivery range between 0.3 and 1.2mg with a systematic manipulation of the nicotine/tar ratio …”

The documents also show the company investigated the pharmacological effects of nicotine. A 1974 memorandum noted: “A general premise in our model of the cigarette smoker is that the smoking habit is maintained by the reinforcing effect of the pharmacologically active components of smoke”. In other words, nicotine.

Last year, in congressional testimony, William Campbell, chief executive of Philip Morris USA, a unit of Philip Morris Companies, said the company does not “manipulate nor independently control the level of nicotine in our products”.

The new documents could be important evidence in the inquiry by the Food and Drug Administration (FDA) into whether the tobacco companies adjust nicotine levels in cigarettes to take account of pharmacological reactions. If they do, the FDA is bound by law to regulate cigarettes, like any official drug.

The FDA chief, David Kessler, is expected shortly to announce his intention to regulate cigarettes. In the face of this threat, and against mainstream scientific evidence, the tobacco companies still deny nicotine is addictive.

Dr Kessler’s declaration would be as important in its way as the landmark 1964 US Surgeon General’s report that declared a causal link between smoking and lung cancer. It would result in a protracted legal battle with the tobacco industry.

Dr Kessler is also expected to seek further restrictions on sales of cigarettes to minors by imposing a ban on cigarette vending machines and a ban on tobacco advertising that portrays smoking as romantic or hip. The legal battle could end with the Marlboro cowboy being driven from the billboards.

The long-expected ruling will hit an industry already besieged by a new raft of multi-million-dollar law suits seeking damages for the effects of tobacco. In an effort to blunt the impact of Dr Kessler’s ruling, tobacco industry spokesmen have already accused him of “creating a subterfuge” in his efforts tocurb teenage smoking and reduce the content of nicotine in cigarettes.