People who have not already developed the condition are unlikely to do so after they quit.

Professor Julian Peto, of the Cancer Research Institute, told a conference in London that the finding indicated a need for new public health strategies.

Currently they focus mainly on dissuading children from starting but fail to encourage adults to quit, he said.

Public health focus

The Cancer Research Campaign backed the call to focus efforts on persuading adults to give up.

The government is due to publish a white paper on public health in the autumn, which will cover strategies to prevent smoking.

Most campaigns aim to prevent youngsters starting

Jean King, head of education for the campaign, said: "We have been pressing the government to put more on cessation in its white paper."

She said: "People should realise that there are benefits at all ages.

"After 10 years the risk of cancer is significantly reduced, and it only takes one year to cut the risk of heart disease."

She added that although nicotine was extremely addictive and hence difficult to give up, there were plenty of support bodies to help smokers quit.

The finding came as the result of long-term research projects such as the 40-year British Doctors Study.

They allow doctors to compare the health of 60-year-olds who stopped smoking at 50 with that of smokers of the same age.

Smoking triggers cancer

The reason for the advantages of quitting applying at any age could lie in the sequential way lung cancer develops.

Tobacco can trigger changes to cells.

A series of these changes must occur to trigger lung cancer, so if someone gives up smoking before all of them have taken place, they are unlikely to develop the disease.

Professor Peto said: "It has not been until the last year that we could see the full horrors of what smoking does."

He said that half of all smokers die as a result of their tobacco use, not a quarter as was previously thought.