Plans announced on Wednesday would give the UK's telecommunications watchdog, Ofcom, power to enforce a "duty of care" on companies such as Facebook and Twitter "to protect users from harmful and illegal terrorist and child abuse content".

Firms that allow harmful material to flourish or don't remove it quickly could be sanctioned.

Ofcom currently keeps tabs on radio and television broadcasters and has the power to levy fines or even kick repeat offenders off the air.

The government said it was "minded" to make the changes but new legislation will be needed for it to take effect. It said officials were working at pace to draft a new law.

Digital Secretary Nicky Morgan said the new rules would be "proportionate and strong".

"We have an incredible opportunity to lead the world in building a thriving digital economy, driven by groundbreaking technology, that is trusted by and protects everyone in the UK," she said.

The National Society for the Prevention of Cruelty to Children said it welcomed "a duty of care model that puts the onus on big tech to prevent online harms".

But free-speech advocates have expressed concerns about state attempts to limit activity that may be harmful but is not illegal.

Australian Associated Press