Ofcom has credited the “Greta effect” after recording an increase in the proportion of 12-15-year-olds taking part in online activism for environmental and political causes.

Almost a fifth of children in the age group gave support to causes or organisations by sharing or commenting on social media posts last year, up by eight percentage points on 2018, and one in 10 signed an online petition. Greta Thunberg began her school climate strikes in August 2018.

In its annual survey on children’s media use, Ofcom also found that more parents than ever believe the internet carries more risks than benefits for children. Hateful material and pressure to make in-app purchases while playing games were among the chief concerns raised by adults.

The proportion of UK parents and carers who believe the advantages of young people being online offset the dangers has fallen by 10 points since 2015 to 55% last year.

More than half of 12-15-year-olds said they seen hateful material – such as social media posts or videos directed towards people based on their identity – online last year.

More than two-fifths of the 3,500 adults in the survey said they were worried about children viewing content that encouraged them to harm themselves.

Yih-Choung Teh, Ofcom’s strategy and research group director, said: “Today’s children have never known life without the internet, but 2 million parents now feel the internet causes them more harm than good. So it’s encouraging that parents, carers and teachers are now having more conversations than ever before with children about online safety.”

Although children are more likely to be trusted with their own smartphone than they were five years ago, many adults have concerns about mobile games, an $86bn (£66bn) global market in 2019.

Nearly half of parents and carers worried that children were being pressured to make in-game purchases of things such as “loot boxes”, virtual items containing rewards.

The report also highlight changes in the way children use the internet. It said an increasing number of older children were using WhatsApp to communicate – 62%, up from 43% the previous year – while one in seven used the video-sharing social network TikTok.

According to Ofcom, the age by which children became digitally dependent has fallen, with half of nine- and 10-year-olds already owning a smartphone, compared with 30% in 2015.