School children can damage their posture with heavy bags (Reuters / Kiyoshi Ota)

Children are setting themselves up for future back problems with heavy school bags, a fresh study has warned.

Research carried out in a Spanish school found that the majority of pupils around the age of 14 were carrying more than 10 percent of their bodyweight in their schoolbags.

The study, published in the British Medical Journal, suggests that children with the heaviest bags were significantly increasing their risks of back problems in the future.

The authors claim that heavy backpacks can alter posture and gait, causing problems as the teenager grows.

Of the 1,403 pupils studied in schools in northern Galicia, 61.4 percent were found to be carrying school bags of a tenth of their weight or more. The average weight of a school bag was found to be around 7kg (15lb).

More than 18 percent of pupils carried a bag that represented more than 15 percent of their body weight, while 10.5 percent reported suffering back pain lasting more than 15 days in the previous year.

"Many children transport excessively loaded backpacks, an excess which would not be allowed for workers," the authors conclude.

"We strongly encourage the medical and educational community to start advising parents and schoolchildren about the risks posed by heavy school bags and the fact that this can be easily reduced."

Experts recommend that youngsters should avoid carrying loads that exceed 10 percent of their body weight. Damage to the back can be mitigated by carrying the bag by straps on both shoulders, rather than one.