Tens of thousands of women have a dramatically increased risk of breast cancer if they have a chest X-ray, according to research.

A study found that women genetically susceptible to breast cancer were 54 per cent more likely to get the disease if they had been given a chest X-ray. If they were younger than 20 when X-rayed, the risk of contracting the disease before the age of 40 increased two and a half times.

The researchers said their results raised questions over the use of mammograms in diagnosing women from families known to have the BRCA 1 and 2 genetic mutations. They said MRI scans - which do not use X-rays - might be a better option.

The BRCA 1 and 2 genes make proteins involved in repairing damage to DNA in breast cells. Mutations to these genes, which affect more than one in 500, leave women with a 40 to 80 per cent chance of developing breast cancer at some point in their lives.

According to the findings, a chest X-ray would see the likelihood of some of these women contracting the disease increase from a "high" chance to "extremely high".

Some experts said that, if confirmed, the study could have "significant practice implications" and "could potentially eliminate" mammographic screening of young women. However, cancer charities warned against spreading alarm among women about having mammograms, saying the study was not "conclusive" as it had been based on people's memories of having had X-rays in the past.

Dr David Goldgar, who led the research at the Genetic Epidemiology Group at the International Agency for Research on Cancer in Lyons, France, said his study was one of the first to demonstrate "that women genetically predisposed to breast cancer may be more susceptible to low-dose ionising radiation than other women".

However, he stressed the need for further study to confirm the work, as it was based on people's recollections of having X-rays.

More than 1,600 women completed questionnaires asking if they had ever had chest X-rays. The researchers excluded mammograms, saying this would prejudice a study based on recollections "because of its obvious relationship to diagnosis". But they said the results "raise the issue of the potential risks of mammographic screening", which uses X-rays.

The research appeared in yesterday's edition of the Journal of Clinical Oncology, and in an accompanying article, Dr Angela Bradbury and Professor Olufunmilayo Olopade wrote: "If confirmed, this study could have significant practice implications for the BRCA mutation carriers and could potentially eliminate mammographic screening as a surveillance method for early detection of breast cancer in young women."

Professor John Toy, the medical director of Cancer Research UK, urged women not to panic. He pointed to a Lancet Oncology study, published earlier this year, which found no association between mammograms and breast cancer among those with the BRCA mutations.

On balance, he said, it was worthwhile to have the screening to allow early detection of cancer - and therefore more effective treatment - despite the risk the mammogram could kick-start the disease. "We must interpret these results with caution. This type of study has inherent limitations because it relies on participants recalling the X-rays they have received ... [and it] looked at chest X-rays and not mammograms," he said.

However, Prof Toy said that if the study was found to be correct, it could lead to more use of MRI scans. He gave as an example an 18-year-old woman with BRCA mutations attending hospital with a broken rib. "The doctor might say, 'I think you've cracked your rib. Normally, we'd perhaps take an X-ray and confirm this, but I don't think you have punctured your lung and you've told me you are a BRCA mutation carrier, so perhaps we'll agree that we will forgo the X-ray'," he said.

Dr Sarah Rawlings, of the charity Breakthrough Breast Cancer, said: "This study does not yet offer conclusive evidence. It's still important for women to attend their breast screening appointments as mammography can detect breast cancer early, when it is more likely to be successfully treated."