Middle-aged people who live alone have double the risk of dementia compared with those who are married or have a partner, researchers have said.

There is a "substantial" link between marital status and dementia and having a partner appears to offer protection against mental decline in later life, they said.

A study showed that people who live alone in middle-age and are widowed or divorced have the highest chances of developing dementia, including Alzheimer's disease.

They are three times more likely to develop dementia, as are people who are single at middle-aged but also when they are older.

Researchers studied 1,449 people aged 50 on average and then again when they were between the ages of 65 and 79. They found that people living with a partner or married in mid-life were less likely than all other categories (single, separated or widowed) to have dementia in later life.

The experts suggested women overall had less chance of dementia than men, but called for more research on differences between the sexes.

The team of researchers was led by Miia Kivipelto from the Karolinska Institute in Stockholm. Writing in the British Medical Journal (BMJ), they said: "Living in a relationship with a partner might imply cognitive and social challenges that have a protective effect against cognitive impairment later in life."

They concluded: "There is a substantial and independent association between marital status in mid-life and cognitive function later in life. People without a partner had twice the risk of developing cognitive impairment and Alzheimer's disease compared with people living with a partner.

"The risk for cognitive impairment was even higher for those without a partner both at mid-life and later life."

The researchers noted that, in 2005, an estimated 25 million people worldwide had dementia, and the number is expected to reach 81.1 million in 2040. In the UK, some 800,000 people have dementia but the figure is rising.