It’s every smoker’s dread — discovering that your genes, as well as your behaviour, put you in the highest category of risk for lung cancer.

But receiving this gloomy diagnosis is the extra kick that many need to quit, according to the developers of a genetic test designed for smokers.

Based on a combination of DNA analysis and non-genetic factors, the Respiragene test gives smokers a susceptibility score for lung cancer ranging from “moderate risk” to “very high risk”. However, geneticists and health professionals have branded the test “bad science”.

“The test is invalidated by any of the normal scientific standards,” Paul Pharoah, a genetic epidemiologist at the University of Cambridge, said.

He warned that there is insufficient proof that all the genetic markers used are associated with an increased risk of lung cancer, and there is only flimsy anecdotal evidence that the test, which costs £299, would help those who wish to give up.

Rob Young, the director of Respiragene, which is based in New Zealand, said that in preliminary trials the test had proved to be highly effective. “Fear is a powerful motivator,” he said.

While lung cancer is extremely rare in non-smokers, affecting about one in 200 people, more than one in 20 smokers will develop the disease.

Receiving a personalised assessment can help smokers to overcome complacency about their risk, according to Dr Young, who says the “it won’t happen to me” mentality is frequently a hurdle to quitting. Smokers can dramatically underestimate the risk of their own likelihood of contracting lung cancer and other smoking-related diseases, he said.

The test uses 20 genetic markers, which Respiragene claims are linked to an increased risk of lung cancer. However, not all of these markers have been extensively validated in the scientific literature.

“Very large genome-wide studies on tens of thousands of people have shown up at most five reliable genetic markers,” Dr Pharoah said.

The 15 other markers used in the test are based on the results of a single study on 439 participants — 200 controls and 239 lung cancer patients — carried out by Dr Young and part-funded by Synergenz BioSciences, the parent company of Respiragene. Dr Pharoah describes this as a “serious conflict of interest”.

“It’s outrageous that they should be selling something based on these results,” he said.

Smokers are required to give their age, the age when they started smoking, their daily intake of cigarettes and their family medical history.

A mouth swab is used to extract a DNA sample, which is sent for laboratory analysis.

About two weeks later a “risk score” is provided. On a scale of 1 to 12, 1-3 corresponds to “moderate risk”, compared to an average smoker; 4-5 is “high risk” and 6-12 “very high risk”. Genetic factors account for about one third of the risk.

Because no “protective” genes are tested for, there is no category that tells smokers that they are at lower-than-average risk of developing cancer.

“We do not want to encourage anyone to believe they are at no risk,” Dr Young said.

The response pack also includes information on smoking and deaths caused by lung cancer and the rates at which susceptibility drops off after people quit. A consultation service is offered where smokers can ring Respiragene to discuss their results.

Since the test was launched, Dr Young said that about 100 tests had been sold in New Zealand. Eventually, he said he hopes to market Respiragene to public health programmes.

Peter Farndon, director of the National Genetics Education and Development Centre, said the technology would need to be more rigorously validated before being considered by the NHS.

“The problem with many of these genetic tests is that they fall down on the science. If it really was an accurate predictor of risk, it would be the sort of thing the NHS would be looking at including in its care pathway.”

He also warned that taking the test may not provide the intended impetus to quit, but may be seen by those in the lowest-risk category as carte blanche to continue smoking, while those at high risk may feel that with such a dire prognosis there was no point in even trying to give up.

“People’s psychological responses can be very unpredictable,” he said.

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