Heavy school bags are causing children high levels of back pain, a study has concluded.

The findings revealed many pupils' backpacks were "excessively loaded", leaving students with back problems which often worsened with age.

The authors of the report, which was published in the Archives of Disease in Childhood, said children should not carry anything weighing more than 10% of their body weight. But the study found nearly two thirds of the 1,403 children surveyed carried bags which broke the "10% rule".

The research, carried out at hospitals in Burela and Palma in Spain, split the students into four groups according to the weight of their bags. The pupils with the heaviest load were 50% more likely to complain of back pain than those with the lightest luggage.

The report added back pain is a bigger problem in girls, with the risk of pain increasing with age.

"The results obtained have strong implications", the report concluded.

"Many children transport excessively loaded backpacks, an excess which would not be allowed for workers in employment."

Back Care's Sean McDougall told the Daily Mail the problem is a "healthcare" timebomb.

"If children are getting back pain so young then there is the potential they will have it for the next 70 to 80 years, which leads to an enormous physical and emotional cost."

McDougall's comments follow the charity's research which claims many children carry as much as 25% of their weight on their backs. The charity urged parents to keep an eye on the amount pupils were taking to and from school.

A study on the management of back pain in children conducted in 2008, which was published in the same archives, said a small proportion of children seek medical advice.

"Although back pain has traditionally been considered a rare and often sinister presentation in the paediatric age group," the study's authors continue, "Recent literature now suggests that a relatively high number of children do experience back pain."