Tobacco companies tried to cast doubt on the link between smoking and cancer by funding projects that challenged the findings of a landmark study, scientists said.

The study concerned the link between tobacco and cancer-causing changes in a gene called p53. In 1996, researchers showed that a chemical in cigarette smoke caused mutations in the gene that were the same as those found in lung cancer tumors.

Scientists at the University of California, San Francisco said the tobacco industry tried to counter the findings for a long time after the study was published,

"The tobacco companies claim they are now working with the public health community 'to support a single, consistent public health message on the role played by cigarette smoking in the development of the disease in smokers,"' said Dr Stanton Glantz.

"But their multifaceted response to p53 research as recently as 2001 suggests that they have not changed their practice."

The Tobacco Manufacturers Association said it was not able to comment on the report.

Damage to p53 leads to uncontrolled cell division. Mutations in the gene are found in more than half of all cancers and 60 percent of lung cancers.

In a report published online by The Lancet medical journal, Glantz and his colleagues examined 43 previously confidential tobacco industry documents about p53 and tobacco smoke.

They said they found evidence that the tobacco industry planned and carried out research programs after the 1996 study to counter the scientific link between smoking and cancer.

"We have identified two instances where research arguing against the connection between tobacco smoke and patterned p53 mutations was undertaken and published by individuals with links to the tobacco companies," Glantz said in the report.

Cancer Research UK, a leading charity, said the study demonstrates that the scientific community must be continually vigilant against the tobacco industry's attempts to influence and distort scientific research.

"Research into p53 is crucial to our understanding how cancer develops and could help us find ways to prevent and treat the disease," Jean King, the charity's director of Tobacco Control, said in a statement.

"Cancer Research UK strongly encourages universities to shun funding from the tobacco industry," she added.