Ofcom is set to be handed new powers to regulate social media companies and protect users from harmful content, the government has announced.
The broadcast watchdog will have its remit expanded to enforce a legal "duty of care" from firms like Facebook, Instagram and YouTube.
It will be tasked with ensuring the platforms protect people from harmful, illegal terrorist and child abuse content by removing it quickly and minimising the risk of it appearing.
But there has been no confirmation of what punishments or fines the bolstered regulator will be able to hand out.
Digital Secretary Baroness Morgan said: "We will give the regulator the powers it needs to lead the fight for an internet that remains vibrant and open but with the protections, accountability and transparency people deserve."
She added the measures would only apply to websites that allow sharing of user-generated content - for example, through comments, forums or videos - meaning fewer than 5% of all UK businesses will be affected.
YouTube reacted by saying the news was of "great importance" and promised to "work in partnership with the government and Ofcom to ensure a free, open and safer internet that works for everyone".
And the Internet Association, which represents firms such as Amazon, Google and Microsoft, said it was keen to debate "issues of concern that are still under review", citing questions over content that is legal but potentially harmful.
Some senior MPs were less than pleased with the announcement, which comes ahead of another response to consultation not released until spring.
Conservative Julian Knight, who chairs the Commons digital committee, said the move "fails to demonstrate the urgency that is required".
He added Ofcom must take a "muscular approach" and be able to punish social media firms "through sanctions that bite" - with "more than a hefty fine".
Tracy Brabin, Labour's shadow digital secretary, claimed it was "shameful" the time to change the law was taking so long.
"Today's proposals are long overdue, and nothing short of legislation will reassure families that their loved ones are safe online," she said.
Concerns about the impact of social media on vulnerable people come amid suicides such as that of 14-year-old schoolgirl Molly Russell in 2017, who was found to have viewed harmful content online.
Her father Ian spoke of the urgent need for greater action, blaming "pushy algorithms" for letting the "wrecking ball of suicide" smash "brutally" into his family.
Ofcom was set up in 2003 and took on duties previously performed by the separate regulators of broadcasting, telecommunications and radio.