The children’s commissioner for England has accused social media companies such as Facebook and Snapchat of losing control of the content carried on their platforms, telling them that recent teen suicides should be a “moment of reflection” for the way they operate.

In an open letter to Facebook, Instagram, WhatsApp, YouTube, Pinterest and Snapchat, Anne Longfield said the suicide of 14-year-old Molly Russell has highlighted the “horrific” material that children were able to easily access online.

“The recent tragic cases of young people who had accessed and drawn from sites that post deeply troubling content around suicide and self-harm, and who in the end took their own lives, should be a moment of reflection,” Longfield told the companies.

“I would appeal to you to accept there are problems and to commit to tackling them – or admit publicly that you are unable to.”

Molly’s father Ian has said that social media was partly to blame for his daughter’s death, after her Instagram account was found to contain distressing material about depression and suicide.

“The potential disruption to all user experiences should no longer be a brake on making the safety and wellbeing of young people a top priority. Neither should hiding behind servers and apparatus in other jurisdictions be an acceptable way of avoiding responsibility,” Longfield added.

Longfield reiterated her call for an independent “digital ombudsman” to ensure that the companies protect young children and speed up the removal of disturbing material.

“I do not think it is going too far to question whether even you, the owners, any longer have any control over their content,” Longfield wrote.

“If that is the case, then children should not be accessing your services at all, and parents should be aware that the idea of any authority overseeing algorithms and content is a mirage.”

A spokesperson for Facebook, which also owns Instagram and WhatsApp, said: “We have a huge responsibility to make sure young people are safe on our platforms and working together with the government, the children’s commissioner and other companies is the only way to make sure we get this right.

“Our thoughts are with Molly’s family and with the other families who have been affected by suicide or self-harm. We are undertaking a full review of our policies, enforcement and technologies and are consulting further with mental health experts to understand what more we can do.”

Longfield’s letter included questions that she wanted answered by the companies, including how many self-harm sites or postings are hosted on their platforms, and how many are accessed by users under the age of 18.

The companies were also asked to reveal the results of their own research into the impact of self-harm sites on children’s mental health, and what support options are offered to users searching for images of self-harm.

“It is your responsibility to support measures that give children the information and tools they need growing up in this digital world – or to admit that you cannot control what anyone sees on your platforms,” Longfield told the companies.

The intervention comes after Ofcom this week said the proportion of 12- to 15-year-olds who reported being bullied over social media rose from 6% in 2016 to 11% last year.

Snapchat said its public content was moderated and “highly curated”, using only content from leading media companies and celebrities, and collected from other users by its in-house news team or professional partners.

“We work hard to keep Snapchat a safe and supportive place for everyone. From the outset we have sought to connect our community with content that is authoritative and credible and safeguard against harmful content and disinformation,” said a spokesperson.

In the UK, Samaritans can be contacted on 116 123 or email jo@samaritans.org. In the US, the National Suicide Prevention Lifeline is 1-800-273-8255. In Australia, the crisis support service Lifeline is 13 11 14. Other international suicide helplines can be found at www.befrienders.org.