Women may need to exercise for an hour day to stop gaining weight, according to a new study.

The study, conducted by Harvard researchers and published in the Journal of the American Medical Association, involved 34,000 women - with a mean age of 54 - from 1992 to 2007.

The women reported their weight and physical activity level at baseline and then at months 36, 72, 96, 120, 144 and 156.

Participants were split into three groups: those who did more than seven hours of exercise a week, those who did between two and a half and seven, and those who did less than two and a half hours.

The mean average weight gain was 2.6kg over the 13 years. Only the women who exercised more than seven hours a week showed no weight gain and there was no difference between the other two groups.

Women who were a normal weight at the beginning of the 13-year period gained less weight the more they exercised, but overweight women saw no correlation between exercise and weight gain.

According to the authors, the study reached two conclusions: “Firstly, once overweight, it may be too late [to prevent weight gain] because physical activity was not associated with less weight gain.

“Secondly, sustaining high levels of physical activity [60 minutes a day] is needed to successfully maintain normal BMI and prevent weight gain.”

Current UK government guidelines that recommend 30 minutes of exercise five times a week are intended to reduce risks to health, not to prevent weight gain.