The study's lead author, Associate Professor Samuel Harvey, said: "We've known for some time that exercise has a role to play in treating symptoms of depression, but this is the first time we have been able to quantify the preventative potential of physical activity in terms of reducing future levels of depression.

"These findings are exciting because they show that even relatively small amounts of exercise, from one hour per week, can deliver significant protection against depression."

An international research team, led by the Australia-based Black Dog Institute, studied 33,908 Norwegian adults whose levels of exercise and symptoms of depression and anxiety were monitored over 11 years.

A healthy group of participants were asked to report how often they exercise and at what intensity, ranging from without becoming breathless or sweating to becoming breathless and sweating or exhausting themselves.

At a follow-up stage, they completed a self-report to indicate any emerging anxiety or depression.