People who are overweight have been getting more so while thin people have changed little, a new study has found.

The research indicates that the average weight of people in Scotland has been pushed up not always by more people growing fat, but by those who are overweight becoming heavier.

Cancer Research UK, which said that people who are overweight were more at risk of the disease, made the findings after comparing statistics over a 10-year period.

Two-thirds of men and almost three-fifths of women are overweight or obese, according to the latest Scottish Health Survey.

The proportion of morbidly obese adults - those most at risk from their size - has doubled since 1995.

Professor Jane Wardle, director of Cancer Research UK's health behaviour unit and study leader, said: "We found weight gain in the population has been unequally distributed.

"Slimmer adults today are almost as slim as their counterparts 10 years ago but the heaviest people in the population are much heavier than they were 10 years ago.

"These inequalities are greatest among those under 45, suggesting that environmental changes are having a greater impact on young adults."

She continued: "Snacking habits, takeaway meals high in fat and sedentary lifestyles where many people spend both work and leisure time sitting in front of a computer are all likely to contribute to the results.

"It seems some people are more susceptible to changes in the environment than others and the explanation for this may be partly genetic.

"It is important for psychologists, geneticists and biologists to work together to discover the reasons for this."

Dr Catherine Hankey, senior lecturer in human nutrition at Glasgow University, said the study showed that the rise in average weight could be attributed at least in part to overweight people who have become more obese.

"What they seem to be saying is that the ones who are thin have stayed thin because they are prepared to take responsibility for managing their weight.

"The overweight people who were overweight have gone to obese and therefore pushed this (average) up for both men and women."

She added: "The first thing people have to do is be aware of what their weight is.

"One of the important things is to regularly weigh yourself. Research shows that you should also be regularly active, and activity should become part of your life, not necessarily going to the gym but walking regularly.

"You have to make your life routinely active rather than sporty active only."

Dr Lesley Walker, director of Cancer Information at Cancer Research UK, said: "We know that high body weight increases the risk of a number of cancers and it is important we get this message out to as many people as possible."

The Scottish Government estimates that nearly half a million cases of high blood pressure north of the border are linked to over-eating.

More than 24,000 heart attacks, 550 strokes and nearly 900 cases of cancer a year are also attributed to the nation's weight problem.

It is also estimated that obese people in Scotland are 18% more likely to be hospitalised than those of normal weight.