Infertility could increase the chance of cancer in mid-life by almost a fifth, research suggests.

The study of more than three million women of childbearing age found that those with fertility problems were 18 per cent more likely to develop the disease.

Scientists from Stanford University said it was not possible to show whether infertility - or treatment for it - was a cause of cancer.

The explanation could be an underlying mechanism which increased the risk of cancer and infertility, they said.

But the four-year study of women who were in their 30s when monitoring began, found that those who suffered infertility problems were 18 per cent more likely to develop any type of cancer during the period.

Researchers stressed that the overall risk of cancer at this stage in life remained low.

Overall, those with fertility problems had an absolute risk of 2 per cent, compared with that of 1.7 per cent among other women.

The findings were published in the journal Human Reproduction.

Study leader Dr Gayathree Murugappan, a fellow in reproductive endocrinology and infertility at Stanford University School of Medicine in California, said: "We do not know the causes of the increase in cancer that we found in this study, whether it might be the infertility itself, the causes of the infertility, or the infertility treatment.