Social media companies that breach new “duty of care” laws should face criminal sanctions punishable by unlimited fines, says Britain’s biggest child protection charity.

The NSPCC will today publish its blueprint for a statutory duty of care on social media firms enforced by a regulator with powers to fine them millions of pounds and sanction criminal investigations if they fail to prevent children being harmed online.

A new offence, modelled on corporate manslaughter laws, would see social media firms prosecuted for a “gross” breach of their duty of care if they had not introduced adequate procedures to protect children from harms including grooming, self-harm, abuse and bullying.

The firms would have to appoint named executives who would be personally liable for ensuring their duty of care was upheld. If found guilty of a breach, they could be banned as directors for up to 15 years using current disqualification laws.

The NSPCC says the sanctions are needed to “embed” regulatory compliance in the firms to prevent a repeat of tragedies such as the death of Molly Russell, 14, who committed suicide after viewing self-harm images. Her father, Ian, blamed Instagram for contributing to her death.