Online forum 4Chan has been fined £450,000 by regulator Ofcom for failing to have age checks in place to stop children from seeing pornography on its site.
The controversial US website must now add an effective age check by 2 April or face a penalty of £500 a day, the online safety watchdog said.
Ofcom has also fined 4Chan a further £50,000 for not assessing the risk of people seeing illegal material on its platform.
It has fined the website an additional £20,000 for failing to set out in its terms of service how it protects users from illegal content.
It must also address these failings by April 2 or face extra daily penalties of £200 and £100 respectively.
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Suzanne Cater, director of enforcement at Ofcom, said: "Companies - wherever they're based - are not allowed to sell unsafe toys to children in the UK.
"And society has long protected youngsters from things like alcohol, smoking and gambling. The digital world should be no different.
"The UK is setting new standards for online safety.
"Age checks and risk assessments are cornerstones of our laws, and we'll take robust enforcement action against firms that fall short."
The fine is the latest one issued to 4Chan by Ofcom as it increases pressure on the US firm to meet its online safety rules.
Last October, Ofcom issued a £20,000 penalty to the group for ignoring requests to share information about the risk of illegal content on its platform.
4Chan was not immediately available for comment.
New online safety protections for children and adults came into force on 25 July last year under the UK's Online Safety Act, which sees new duties placed on platforms to take down illegal and harmful content.
In August last year, 4Chan launched a legal case against Ofcom in the US over claims it tried to "censor" the website.
(c) Sky News 2026: 4chan website fined by Ofcom for failing to protect children from seeing pornography
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