LONDON (AP) -- People with early lung cancer who quit smoking could double their chances of surviving, a new study says.

Until now, there has been little proof that quitting smoking after developing lung cancer makes any difference to survival.

British researchers analyzed previous data from 10 studies examining how long smokers survived after being diagnosed with lung cancer.

People with lung cancer who continued smoking had a 29 to 33 percent chance of surviving five years. But those who kicked the habit had a 63 to 70 percent chance of being alive after five years. The research was published Friday in the BMJ, formerly known as the British Medical Journal.

Lung cancer is the top cancer worldwide, and the prognosis is usually poor. Only about 7 percent of patients make it to five years, though about 20 percent of patients are diagnosed early enough to be treated.

"The message is you should never give up on giving up (smoking)," said Amanda Parsons, of the U.K. Centre for Tobacco Control Studies at the University of Birmingham, who led the study. "Even at the stage where you have been diagnosed with early stage lung cancer ... if you give up smoking, your body can still partially recover and your risk is reduced," she said.

While some doctors recommend lung cancer patients quit smoking, not all do. Some doctors and nurses "think it is inhuman to dwell on the matter -- that it adds to feelings of guilt and takes away a lifelong comfort from the dying patient," wrote Tom Treasure of University College London and Janet Treasure of King's College London in an accompanying editorial in the BMJ.

They said patients and their families should now be told about the study results, "because the potential benefit is great."

The research might also provide some clues on how smoking causes cancer. Scientists aren't sure if tobacco smoke or nicotine affect lung cancer once it has developed, though there is some evidence they may speed up the disease. Knowing how cigarettes impact cancer could potentially lead to new treatments, Parsons and colleagues wrote.

The study was paid for by the British Heart Foundation, Cancer Research U.K. and other governmental bodies.

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