A drug that frees the immune system to attack a devastating form of lung cancer has been shown to double the life expectancy of genetically targeted patients.

Nivolumab is one of new generation of immunotherapy drugs that release cancer-applied brakes on the immune system called "checkpoints".

The results, from a major international trial involving patients who had already been treated for the most common form of lung cancer, were described by one expert as a "paradigm shift".

In the Phase III trial, the last step before a drug is licensed for use in clinics, researchers compared the effectiveness of nivolumab and the standard chemotherapy drug docetaxel in 582 patients with advanced non-squamous non-small cell lung cancer.

The disease accounts for around 85 per cent of all cases of lung cancer.

Overall, nivolumab reduced the risk of dying by 27 per cent compared with docetaxel and increased typical survival time from 9.4 to 12.2 months.

But the drug was found to be most effective in patients whose cancers produced higher levels of a tumour protein called PD-L1, potentially paving the way to personalised treatments.

For those with the most active PD-L1 gene in their cancer cells survival time more than doubled from eight to 19.4 months.

Lower "expression levels" led to life extensions of 10 months and eight months as amounts of the molecule reduced, while patients with little or no PD-L1 saw no survival benefit.

Almost 80 per cent of patients had measurable levels of the protein.

The trial, conducted in North America and Europe, was halted early on ethical grounds because of the strong results. It was led by Dr Luis Paz-Ares, from the Hospital Universitario Virgen Del Rocio, in Seville, Spain.

Dr David Chao, consultant medical oncologist at the Royal Free Hospital, London, said: "This announcement marks a paradigm shift in the treatment of lung cancer".

"For patients that have limited treatment options, it is very encouraging to see the early benefits immunotherapies such as nivolumab can have, and is just the beginning of the journey to further improve and refine these new treatments."

Dr Alan Worsley, senior science information officer at Cancer Research UK, said:

"Harnessing the power of our immune system to fight cancer will be an essential part of future treatments".