A world-famous British scientist failed to disclose that he held a paid consultancy with a chemical company for more than 20 years while investigating cancer risks in the industry, the Guardian can reveal.

Sir Richard Doll, the celebrated epidemiologist who established that smoking causes lung cancer, was receiving a consultancy fee of $1,500 a day in the mid-1980s from Monsanto, then a major chemical company and now better known for its GM crops business.

While he was being paid by Monsanto, Sir Richard wrote to a royal Australian commission investigating the potential cancer-causing properties of Agent Orange, made by Monsanto and used by the US in the Vietnam war. Sir Richard said there was no evidence that the chemical caused cancer.

Documents seen by the Guardian reveal that Sir Richard was also paid a £15,000 fee by the Chemical Manufacturers Association and two other major companies, Dow Chemicals and ICI, for a review that largely cleared vinyl chloride, used in plastics, of any link with cancers apart from liver cancer - a conclusion with which the World Health Organisation disagrees. Sir Richard's review was used by the manufacturers' trade association to defend the chemical for more than a decade.

The revelations will dismay scientists and other admirers of Sir Richard's pioneering work and fuel a rift between the majority who support his view that the evidence shows cancer is a product of modern lifestyles and those environmentalists who argue that chemicals and pollution must be to blame for soaring cancer rates.

Yesterday Sir Richard Peto, the Oxford-based epidemiologist who worked closely with him, said the allegations came from those who wanted to damage Sir Richard's reputation for their own reasons. Sir Richard had always been open about his links with industry and gave all his fees to Green College, Oxford, the postgraduate institution he founded, he said.

Professor John Toy, medical director of Cancer Research UK, which funded much of Sir Richard's work, said times had changed and the accusations must be put into context. "Richard Doll's lifelong service to public health has saved millions of lives. His pioneering work demonstrated the link between smoking and lung cancer and paved the way towards current efforts to reduce tobacco's death toll," he said. "In the days he was publishing it was not automatic for potential conflicts of interest to be declared in scientific papers."

But a Swedish professor who believes that some of Sir Richard's work has led to the underestimation of the role of chemicals in causing cancers said that transparency was all-important. "It's OK for any scientist to be a consultant to anybody, but then this should be reported in the papers that you publish," said Lennart Hardell of University Hospital, Orebro.

Sir Richard died last year. Among his papers in the Wellcome Foundation library archive is a contract he signed with Monsanto. Dated April 29 1986, it extends for a year the consulting agreement that began on May 10 1979 and offers improved terms. "During the one-year period of this extension your consulting fee shall be $1,500 per day," it says.

Monsanto said yesterday it did not know how much work Sir Richard did for the company, but said he was an expert witness for Solutia, a chemical business spun off from Monsanto, as recently as 2000.