Infertile men may be twice as likely to develop breast cancer than those without fertility issues, according to one of the largest ever studies of the disease.

Breast cancer in males is less common than in females and its relation to infertility had previously been investigated only in small studies. The new research was published in the journal Breast Cancer Research.

Experts at the Institute of Cancer Research, London (ICR) said the findings indicated further work was needed to understand the underlying causes of male breast cancer, which remain largely unknown.

“These are important findings linking infertility to breast cancer in men,” said the study’s author, Dr Michael Jones, a senior staff scientist in genetics and epidemiology at the ICR. “Our study suggests that infertile men may be twice as likely as those without fertility issues to develop breast cancer.

“The reasons behind this association are unclear, and there is a need to investigate the fundamental role of male fertility hormones on the risk of breast cancer in men. We hope this could lead to insights into the underlying causes of male, and possibly even female, breast cancer.”

The new research from the Breast Cancer Now male breast cancer study looked at 1,998 men newly diagnosed with the disease in England and Wales over a 12-year period. It is one of the largest studies ever undertaken on male breast cancer.

About 370 men are diagnosed with breast cancer each year in the UK, and because male breast cancer is rare, research into the disease is usually limited to a small number of patients. Studying a larger group of men enabled the team to show a statistically significant association between infertility and risk of male breast cancer.

The men were asked whether they had biological children, if they or their partners had ever experienced problems conceiving, or if they had visited a doctor or clinic for fertility concerns.

Researchers directly compared the fertility of the men with breast cancer with 1,597 men with no history of the disease. While the biological reason is unclear, they discovered that men diagnosed with breast cancer were more likely to report fertility issues.

Scientists also found there were significantly more men with no children among those who had been diagnosed with breast cancer.

Dr Simon Vincent, the director of research, support and influencing at Breast Cancer Now, said: “Many people don’t realise that men can get breast cancer, because incidence is much lower in men than women. However, every year in the UK around 370 men are diagnosed with breast cancer, and around 80 men die from it and it’s vital that we support anyone affected by breast cancer.

“Discovering a link between infertility and male breast cancer is a step towards us understanding male breast cancer and how we could find more ways to diagnose and treat men – and possibly women – with this devastating disease.”