Women who worked long hours, defined as between 41 and 48 hours, or very long hours, meaning more than 49 hours a week, were also more likely to smoke, drink at risky levels and not exercise. About 65 per cent who worked long hours drank at risky levels, compared with 42 per cent of the women who were not in the labour force and 53 per cent who were unemployed.

''These statistics … provide some clues as to how employment patterns may affect lifestyle choices, and subsequently, body weight,'' Dr Au wrote in the International Journal of Obesity.

Fifty-five per cent of the women gained weight over two years and 31 per cent lost weight. "On average the women gained 1.5 per cent of their initial weight but we were seeing quite extreme amounts of weight gain as well.''

Dr Au said said the effect of long work hours was particularly noticeable among the women who gained the most weight, with some gaining nearly 12 per cent of their body weight - the equivalent of about eight kilograms for a woman weighing 69 kilograms.

The finding could have serious health implications because larger changes in weight were likely to have more serious health impacts, Dr Au said. It was important for women to be aware that if they were working very long hours that their health could be at risk.