Schools are being urged to buy bigger chairs because of booming obesity rates among children.

Many young people in Britain are now too fat for classroom furniture that has remained almost unchanged since the 1960s, it is claimed.

A policy commission - led by Charles Clarke, the former Education Secretary - said growing numbers of children were also suffering from back pain due to unsuitable school chairs.

Most schools are continuing to stock cheap furniture built to 50-year-old specifications to save cash, it suggests.

It comes despite the fact that children are likely to spend longer sitting in front of computers, causing discomfort for thousands of pupils.

Headteachers are now being urged to stock chairs to new, bigger European specifications.

According to official figures, almost a quarter of infants and a third of children aged 11 are officially overweight as obesity rates have doubled in just over a decade.

Dominic Savage, director general of the British Educational Suppliers Association, which commissioned the study, said: "Although our starting point was not a question of obesity, when we looked, it is very much that the average child today is very different to a child in the 1960s, which is the last time children were actually measured for determining measures of furniture.

"They are very different in many ways. Obesity or weight is an issue as is the variety of weight of any one cohort of children."

Speaking as the document was launched in central London, Mr Savage said children were also taller and their leg and arm lengths were now different.

New European Furniture Standards - introduced to meet changing body sizes - have been used in a small number of schools, although the BESA report warned that "the education system has been slow to adopt the new standard".

"Our children are likely to be spending thousands of hours of their school lives on chairs and at desks and tables where their posture is poor and the potential for damage to backs is great," said the study.

It admitted that changing all chairs immediately would be "impractical" and expensive, but urged schools to make "greater use of adjustable furniture" to allow children to adapt chairs and tables to their size.