Lung cancers in ex-smokers can lie dormant for as long as 20 years before they begin to grow into a life-threatening tumour, a study has found.

Scientists found that the original mutations in lung cells caused by inhaling cigarette smoke can date back many years before additional mutations cause them to become aggressive cancer cells.

The researchers carried out a genetic analysis of the tumour cells from seven lung-cancer patients and found a surprising variation within each tumour in terms of the DNA mutations that triggered the disease.

Lung cancer has a low rate of survival, with just 10 per cent of patients still being alive five years after diagnosis and being able to understand the genetic evolution of tumour cells will help to improve treatments, scientists said.

“Survival from lung cancer remains devastatingly low with many new targeted treatments making a limited impact on the disease,” said Professor Charles Swanton at Cancer Research UK’s London Research Institute and UCL Cancer Institute.