A growing number of women are going through an early menopause, according to a new study.

Researchers have found that more than one in 20 women went through an early menopause which puts them at greater risk of heart attack, stroke and bone disease in later life.

The Imperial College London study found a higher than expected number stop having periods before the age of 40 for no known medical reason.

Some 6% had unexplained premature menopause and a further 2% had gone through an early menopause due to cancer treatment or surgery to remove the ovaries.

Women from the lowest social class were almost three times as likely to experience menopause before the age of 40.

At present, the average age of menopause in the UK is 51.

Researchers looked at the records of nearly 5,000 women born in Britain in a single week in 1958 and followed up with them eight times. At the age of 50, they asked them about the date and cause of their menopause and their quality of life.

A total of 7.4% had gone through the menopause before the age of 40, with smokers and women from the lowest social class most at risk.

Dr Rumana Islam, who carried out the study with Dr Rufus Cartwright, said despite the findings, it was difficult to explain why early menopause is more common in poorer women, saying factors such as smoking, obesity and lack of physical exercise had already been taken into account and social class remained as an independent risk factor.