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The blood vessels of obese teenagers look more like those found in middle-aged people, say Canadian researchers.

A study of 63 children, whose average age was 13, found signs of "stiffening" in the aorta - the largest artery in the body.

The British Columbia Children's Hospital team said it was an early indicator of heart disease.

The British Heart Foundation described child obesity as a "ticking public health time bomb".

One of the key changes in heart disease is the hardening of arteries supplying blood to the heart.

The rate of childhood obesity has rocketed in the last two decades and continues to increase, leading to fears that younger and younger people will fall prey to heart attacks and strokes, as well as other diseases such as Type 2 diabetes.

Measurements of the elasticity of the aorta were taken using ultrasound, which can help doctors work out how fast blood is flowing through the vessel.

However, when the results for the obese teenagers were compared with 55 children with normal weights, the differences were clear.

More worryingly, these findings were not echoed by similar differences in blood pressure, and blood cholesterol levels between the obese and normal weight children.

This suggests that cardiovascular problems which could threaten or even shorten life could be developing "silently" during childhood.

Dr Kevin Harris, one of the researchers presenting the research at the Canadian Cardiovascular Congress, said: "The systolic blood pressure was only marginally higher in these obese children - but aortic stiffness is associated with cardiovascular events and early death."

He now plans research to see if these aortic changes can be reversed with improved diet and exercise.

Dr Beth Adamson, from the Canadian Heart and Stroke Foundation, said: "We must rethink the lifestyle standards we have accepted as a society to protect the future health of our kids."

Obesity rates in the UK do not lag far behind those in Canada and the US, and the British Heart Foundation said it was equally concerned.

June Davison, a senior cardiac nurse, said: "Last year, figures showed more than a fifth of youngsters are obese or overweight when they start school - a shocking reality and a warning that we are sitting on a ticking public health time bomb.

"Obese children can become obese adults which can mean a whole host of health problems later in life including heart and circulatory disease, the UK's biggest killer.

"We must try and stop the clock ticking by making it easier for families to make informed, healthier choices about the food they're eating and the amount of physical activity they're doing."