The Campaign for a Commercial-free Childhood claim the social network's mooted plan to develop a platform for under-13s is dangerous for their wellbeing and privacy.

In an open letter, the group of 99 public health advocates and organisations warn that the platform would likely be used by young children.

It is believed that Instagram is already popular with under-13s, despite an age limit on the platform.

In March, Buzzfeed reported that, according to an internal company post, Facebook had identified "youth work" as a priority for the image-sharing service. Vishal Sah, Instagram's vice president of product, outlined plans to build "a version of Instagram that allows people under the age of 13" to use it safely for the first time.

Facebook told the BBC that under-13s access to Instagram would be "managed by parents", adding that:

"Kids are already online, and want to connect with their family and friends, have fun, and learn. We want to help them do that in a safe and age-appropriate way, and find practical solutions to the ongoing industry problem of kids lying about their age to access apps."

Facebook stated that their priority is children's safety and privacy, and that they would build the product in consultation with experts on child development, safety, and mental health. The planned app would also be ad-free.

Despite Facebook's assurances, the Campaign for a Commercial-Free Childhood remains unconvinced. The letter states that "a growing body of research demonstrates that excessive use of digital devices and social media is harmful to adolescents", adding that Instagram's "relentless focus" on self-presentation and branding is a threat to children's wellbeing.

It goes on to state: “According to your congressional testimony, Instagram for kids is in the early planning stages. We urge you to abandon those plans. Doing so would send a strong message that Facebook understands the vast concerns about the effects its business model is having on young people, and is open to solutions that truly benefit children and teens, not just Facebook’s market share.”

The letter also highlights Facebook's patchy track record when it comes to privacy for kids, citing a design flaw in Messenger Kids allowed children to circumvent parental controls.

Facebook's rival Alphabet, the owner of YouTube and Google, has also been called out for its practices. YouTube for Kids, for instance, has been criticised for inappropriate product placement.