LONDON, (Reuters) - Britain's government intends to make it illegal to encourage others to harm themselves online and will fine social media companies that fail to remove such material, as part of a revamp of legislation governing online behaviour.

Promoting suicide is already illegal, but Britain's digital, culture, media and sport ministry said in a statement that it now wanted to require social media firms to block a wider range of content.

"Social media firms can no longer remain silent bystanders ... and they'll face fines for allowing this abusive and destructive behaviour to continue on their platforms under our laws," Digital Secretary Michelle Donelan said.

The Conservative government said the proposals aimed to block images and videos similar to those viewed by Molly Russell, a 14-year-old whose death in 2017 sparked ongoing public concern.

In September, the coroner investigating her death ruled that social media platforms had fed content to her which "romanticised acts of self-harm by young people".

Under the proposals, social media companies will have to remove and limit users' exposure to material that deliberately encourages people to harm themselves.

Last week the government said the new legislation would also ban the distribution of sexually explicit images that have been manipulated to look like they feature someone who has not consented to appear in them.

Full details of the latest proposals - including the criminal penalties faced by people who promote self-harm, and the scale of fines faced by companies - will come next month when legislative amendments are put before parliament.

The wider legislation incorporating such penalties, known as the Online Safety Bill, has had a slow passage through parliament since its first draft in May 2021.

Earlier versions sought to ban "legal but harmful" material online, drawing criticism from tech companies and free-speech campaigners who said the definition was too vague and could be used to arbitrarily criminalise otherwise legal behaviour.

However, the bill has been strongly supported by children's and mental health charities, and by people seeking to limit racist and sexist abuse online.