Major surgery doubles the chance of experiencing a substantial decline in mental skills, such as reasoning, memory and language ability, a new study suggests.

Doctors and scientists have long feared that general anaesthetic, mini-strokes or inflammation may damage the brain during surgery, but there has been little evidence to show a long-term impact.

Now a longitudinal study of more than 7,000 British civil servants who were tested between 1997 and 2016 found those who underwent major operations were twice as likely to suffer substantial cognitive decline compared to those who never needed surgery.

While around one in 40 people suffered a significant reduction in mental ability during the 19 year follow-up period, the number rose to more than one in 18 for those who had a major operation.

Major surgery was also found to age the brain by an average of four months and three days.

Although the research did not look at dementia, cognitive decline can precede or speed up the development of dementia because of a loss of brain resilience.

Researchers from the University of Wisconsin said it was unclear whether the effect was driven by the operation itself, or the underlying medical condition that had necessitated the surgery.