Sir Richard Doll has ended a research study that decisively proved tobacco leads to lung cancer, 50 years after the work began.

The research, which has saved tens of millions of lives, began as an investigation into the smoking habits of 40,000 doctors in 1951.

But the study continued for half a century, long after the breakthrough on the lethal effects of tobacco.

Sir Richard, 89, said: "I have just written to the 6,000 surviving doctors to thank them for their assistance over the years and to give them some information about the many papers that they have contributed to. We've decided that we have taken it as far as it can go."

Sir Richard, a former Oxford University Regius Professor of Medicine, arrives for work each day at Oxford's clinical trial service unit, near the Radcliffe Infirmary.

Information from the doctors has continued to provide Sir Richard and his team with data about diseases including cancer and chronic bronchitis, and the beneficial effects of stopping smoking.

The study was extended to cover doctors' drinking habits, providing surprising insights into the risks, and benefits, of alcohol consumption.

Sir Richard did not initially see tobacco as a likely cause of lung cancer.

He said: "I actually thought the rise in lung cancer was something to do with tar on the roads."

The research initially looked at lung cancer patients in 20 hospitals. The revelation that 80 per cent of the adult male population was indulging in a deadly habit was met by a wave of apathy.

Doctors were chosen for a bigger study because, in 1951, 65 per cent of them smoked, and it was comparatively straightforward to keep track of them.

In 1951, Sir Richard wrote to 200 doctors, selected at random. Ultimately 34,000 male doctors and 6,000 women doctors agreed to take part in the study, which established Sir Richard as the world's most celebrated cancer expert.