Some women are undergoing unnecessary mastectomies after taking private DNA tests that show they carry mutated genes that increase the likelihood of developing breast cancer, research suggests.

These gene mutations disproportionately affect Ashkenazi Jews, with around one in 40 people of Ashkenazi Jewish descent carrying a faulty BRCA1 or BRCA2 gene, compared with around one in 400 people in the general population.

Most cases of breast and ovarian cancer are not due to a genetic fault, but where testing does detect this mutation, clinicians can recommend breast or ovary removal surgery as a preventative measure.

Until recently, women who learned they carried a heightened risk did so because they attended a clinic due to symptoms or a family history of disease. However, many people now pay for at-home DNA testing kits, or are given their results after taking part in genetic research, and have not had any personal link with breast cancer, according to a new study.

Scientists at the University of Exeter analysed the data of more than 450,000 people aged between 40 and 69 and found that carrying a disease-causing BRCA variant was linked to a breast cancer risk of 18 per cent for the BRCA2 gene and 23 per cent for BRCA1 by age 60.

When a close relative has had the condition that risk rises to 24 per cent and 45 per cent respectively, according to the research, published in eClinical Medicine, an open access clinical journal, published by The Lancet.

Women informed outside a clinical setting that they carry a disease-causing BRCA variant may be told that their risk of breast cancer is as high as 80 per cent, it said.

In recent years, more women with a faulty BRCA gene have chosen to have surgery, particularly since actress Angelina Jolie (inset) shared her family history of the disease and subsequent genetic test and surgery, the study’s lead author, Dr Leigh Jackson, noted.

But he said: “Being told you are at high genetic risk of disease can really influence levels of fear of a particular condition and the resulting action you may take.

“Up to 80 per cent risk of developing breast cancer is very different from 20 per cent. That difference could well influence the decision you make around whether you have invasive breast surgery… we want [patients] to make an informed decision.”

The research team urged anyone communicating cancer risk to do so “based on a detailed family history, not just genetics alone”.