They say that these findings add to the already heavy burden of public health consequences from type 2 diabetes, and back the urgent need for prevention and control of type 2 diabetes in women.

Risk of the disease can be prevented by dietary intervention.

Researchers interviewed 18,999 women aged 70-81 years, who were taking part in the nurses' health study in the United States. The women were tested for cognitive function and cognitive decline, an intermediate stage between normal ageing and dementia, over two years.

At the start of the study, women with type 2 diabetes performed worse on all cognitive tests than women without diabetes. Odds of poor cognition were 50 per cent higher for women who had had diabetes for more 15 years or more, they report in an early online issue of the British Medical Journal (doi:10.1136/bmj.37977.495729).

In contrast, women with diabetes who were on oral antidiabetic drugs performed similarly to women without diabetes, while women not using any medication had the greatest odds of poor performance compared with women without diabetes.

Diabetes has already increased by one-third during the 1990s, due to the prevalence of obesity and an ageing population. There are currently more than 194 million people with diabetes worldwide but if nothing is done to slow the epidemic, the number will exceed 333 million by 2025, according to the International Diabetes Federation.