As children go back to school for the new term, they head off each morning with bags crammed full of books, sports gear, food and probably CDs and personal stereos.

While carrying a heavy bag may seem harmless enough, experts have warned that it may actually be a major cause of spinal injuries and painful neck and back problems in later life.

An x-ray pictured shows the effect of a heavy bag

Tone Tellefsen, of the British Chiropractic Association, said: "In English studies, they have found that children are regularly carrying 20 per cent of their body weight, which is far too much pressure on growing bones.

"We are seeing a lot of children with back pain -- sometimes quite severe. It stops them playing sport or, at the very least, stops them playing as well as they could do."

As well as problems such as rounding of the shoulders and muscle strain, Mr Tellefsen also said children are having to take days off school as carrying a heavy weight over the shoulders can cause headaches -- the shoulder blade hangs from the skull and so a large load leads to muscle tension.

Sue Shillabeer, of the charity BackCare, said the incidence of back injuries in children was increasing.

"By the age of 14, more than half will have had back pain and some of those will have a problem for life.

"We are just building up problems for the future. Even now, the costs of back pain to the country in terms of days off work are exorbitant and that is set to increase."

For Ms Shillabeer, fewer schools providing lockers is putting greater pressure on children's backs. They are forced to carry their heavy loads not only to and from school, but also between different rooms during the school day.

She believes there should be health and safety regulations to protect children.

"If you put a 16-year-old in the workplace then there is legislation governing how much they can carry, but the same does not apply in schools -- there is a bit of an anomaly in the law there.

"Changing lesson structures so that rather than having eight single lessons in a day, they have four doubles, would reduce the number of books that children have to carry.

"And making sure music lessons and sports lessons are not on the same day would help."

Forcing one shoulder to take the strain forces the backbone to lean to one side, which may cause curvature of the spine and serious problems later in life.

And the uneven pressure on muscles may cause strains, spasms and even the muscles developing more on one side of the body.

Heaviest objects should be packed first so they are carried lower down and close to the body, and a backpack should be picked up with knees bent.

The bottom of the pack should rest a couple of inches above the waist. Ideally, a pack should be made of lightweight material, have padded, adjustable shoulder straps, a waist strap and a padded back.

Carrying heavy backpacks incorrectly is definitely one of the major causes of back pain in children, but it would be wrong to assume it is the only cause.

Jane O'Connor, a practising osteopath, said: "It's just one thing after another -- heavy bags, sitting in the same position at school, sitting at home playing computer games. Children are not using their body as much anymore, so it is not developing in a natural way."

Whatever the cause of pain, most youngsters should not be suffering back problems, so a visit to your GP is advisable.

For more information about back injuries in children, contact BackCare on 0208 966 5474 or visit www.backpain.org