Websites will have to change the algorithms that recommend content to young people and introduce beefed-up age checks or face big fines, the UK media regulator has confirmed.
Ofcom says its "Children's Codes" - the final versions of which have now been published - will offer "transformational new protections".
Platforms which host pornography, or offer content which encourages self-harm, suicide or eating disorders are among those which must take more robust action to prevent children accessing their content.
Ofcom boss Dame Melanie Dawes said it was a "gamechanger" but critics say the restrictions do not go far enough and were "a bitter pill to swallow".