Around one in three boys have been exposed to content encouraging them to build their bodies up, with many urged to buy substances that might not be safe, according to a survey.

A study of nearly 15,000 UK schoolchildren suggested the number viewing potentially harmful online content has increased over the last four years, with particular concern relating to body image.

The research, carried out by Youthworks and Internet Matters, uncovered a worrying new trend among children – particularly boys – aspiring to muscular bodies and six packs.

Those aged 13 were the most likely group to be viewing such content.

Among boys, those who said they “often” looked at material online encouraging them to bulk up their bodies had lower self-esteem than those who never saw this type of content, the report claims.

Meanwhile, more than a quarter of girls (28%) said they had seen sites or messages that “pressure me to be too thin”.

One in four children (25%) aged between 11 and 17, said they had seen pro-suicide content, and one in eight (13%) reported content about self-harm.

“The big message from this report is that harmful content has overtaken cyberbullying as a major threat to young people,” said Adrienne Katz, from Youthworks, who co-authored the report.

“Messages, comments, adverts and ideal bodies seen online can combine to make teenagers obsess about their bodies.

“It is all too easy to buy products promising perfection.

“Given how much they love and benefit from the online world, we owe it to young people to help make it a safer experience.

“Outdated online safety advice is not going to work in this new decade, and suicide content should be rapidly addressed.”

The report, titled In Their Own Words – The Digital Lives Of Schoolchildren, also found positive experiences online.

More than eight out of 10 (84%) said their online life helped them relax after school, while more than half (52%) indicated that their web activity has helped them find and talk to people like themselves most or some of the time.

Carolyn Bunting, chief executive of Internet Matters, said: “Harmful content has become one of our biggest concerns in the online space, with numbers of children viewing this material increasing over the last four years.

“With too few children following online safety advice they’ve been taught in school or from their parents, especially as they get older, a new dialogue is needed for children in their mid-teens so we can engage them more with online safety.

“No parent wants their child to be making life-changing decisions having been exposed to this content, so it’s vital parents get involved with their children’s digital lives to understand what they are seeing and provide support where needed.”