The controversial online forum 4chan risks being blocked in the UK after refusing to pay a fine from Ofcom for breaking rules set by the Online Safety Act.
The website, which hosts adult content, was handed a £20,000 for failing to share information about the risk of illegal content on its platform.
Media watchdog Ofcom said action would be taken against companies that "flagrantly fails to engage with Ofcom and their duties under the Online Safety Act".
Lawyers for 4chan described the fine, which is the first to be handed down under the new regulation, as “risible” and “stupid”.
4chan, which has been referred to as the “cesspit of the internet”, has previously been blamed for radicalising young men, while also being linked to mass shootings.
It is also the founding place of the online hacking collective Anonymous, and the birthplace of some of the internet’s most popular memes, such as Pepe the Frog and Rickrolling.
Websites that fail to comply with the Online Safety Act face financial penalties of up to £18 million, or 10 per cent of worldwide turnover. If these are ignored, courts could require internet service providers to block access to the site.
“Today sends a clear message that any service which flagrantly fails to engage with Ofcom and their duties under the Online Safety Act can expect to face robust enforcement action,” said Suzanne Cater, Director of Enforcement at Ofcom.
“We're also seeing some services take steps to introduce improved safety measures as a direct result of our enforcement action. Services who choose to restrict access rather than protect UK users remain on our watchlist as we continue to monitor their availability to UK users.”
The rules came into force in July, aimed at protecting children from harmful content. They require sites hosting adult material to perform robust age verification checks, such as requesting government-approved photo ID, checking credit card information, or using facial age estimation technology.
4chan has 60 days to provide the information requested by Ofcom, with an additional fine of £100 added for each day that it is ignored.
The site has filed its own lawsuit against Ofcom, accusing the regulator of overreaching its legal authority by applying UK laws to companies based in other jurisdictions.
“That Americans don’t obey British censors is a matter of settled US law,” Preston Byrne, managing partner of law firm Byrne & Storm, which represents 4chan, told The Telegraph.
“Maybe instead of sending my clients stupid letters, they should think about answering the lawsuit we filed against them in DC federal court.”
Technology Secretary Liz Kendall said in a statement: "Services can no longer ignore illegal content, like encouraging self-harm or suicide, circulating online which can devastate young lives and leave families shattered.
"This fine is a clear warning to those who fail to remove illegal content or protect children from harmful material.”