The online message board 4chan is being investigated by the UK communications regulator over failure to comply with recently introduced online safety rules.
Ofcom says it has received complaints over potential illegal content on the website, which has not responded to its requests for information.
Under the Online Safety Act, online services must assess the risk of UK people encountering illegal content and activity on their platforms, and take steps to protect them from it.
Ofcom is also investigating porn provider First Time Videos over its age verification checks, and seven file sharing services over potential child sexual abuse material.
4chan has been contacted for comment.
Ofcom says it requested 4chan's risk assessment in April but has not had any response.
The regulator will now investigate whether the platform "has failed, or is failing, to comply with its duties to protect its people from illegal content".
It would not say what kind of illegal content it is investigating.
Ofcom has the power to fine companies up to 10% of their global revenues, or £18m - whichever is the greater number.
4chan has often been at the heart of online controversies in its 22 years, including misogynistic campaigns and conspiracy theories.
People are anonymous, which can often lead to extreme content being posted.
It was the subject of an alleged hack earlier this year, which took parts of the website down for over a week.
Seven file sharing services also failed to respond to requests for information from the regulator.
They are Im.ge, Krakenfiles, Nippybox, Nippydrive, Nippyshare, Nippyspace and Yolobit.
Ofcom also says it has received complaints over potential child sexual abuse material being shared on these platforms.
Separately, porn provider First Time Videos, which runs two websites, is being investigated into whether it has adequate age checks in place to stop under-18s accessing its sites.
Platforms which host age-restricted content must have "robust" age checks in place by July.
Ofcom does not specify exactly what this means, but some platforms have been trialling age verification using facial scanning to estimate a user's age.
Social media expert Matt Navarra told BBC News earlier this year facial scanning could become the norm in the UK.
Sign up for our Tech Decoded newsletter to follow the world's top tech stories and trends. Outside the UK? Sign up here.
Ofcom previously found the corporation breached its operating licence by cutting about 300 hours of news and current affairs.
The EU said the sites did not appear to have "appropriate" age verification to stop children accessing porn.
The company has already faced a £16m penalty for failing to deliver post on time over the previous two years.
The UK demanded in January that porn sites show how they were beefing up age verification to stop children accessing them.
It says the 2024 summer riots showed the regulator does not have the tools to stop the spread of viral misinformation.
Copyright 2025 BBC. All rights reserved. The BBC is not responsible for the content of external sites. Read about our approach to external linking.