A lawyer representing the online message board 4chan says it won’t pay a proposed fine by the UK’s media regulator as it enforces the Online Safety Act.

According to Preston Byrne, managing partner of law firm Byrne & Storm, Ofcom has provisionally decided to impose a £20,000 fine “with daily penalties thereafter” for as long as the site fails to comply with its request.

“Ofcom’s notices create no legal obligations in the United States,” he told the BBC, adding he believed the regulator’s investigation was part of an “illegal campaign of harassment” against US tech firms.

Ofcom has declined to comment while its investigation continues.

“4chan has broken no laws in the United States – my client will not pay any penalty,” Mr Byrne said.

Ofcom began investigating 4chan over whether it was complying with its obligations under the UK’s Online Safety Act.

Then in August, it said it had issued 4chan with “a provisional notice of contravention” for failing to comply with two requests for information.

Ofcom said its investigation would examine whether the message board was complying with the act, including requirements to protect its users from illegal content.

4chan has often been at the heart of online controversies in its 22 years, including misogynistic campaigns and conspiracy theories.

Users are anonymous, which can often lead to extreme content being posted.

In a statement posted on X, law firms Byrne & Storm and Coleman Law said 4chan was a US company incorporated in the US, and therefore protected against the UK law.

“American businesses do not surrender their First Amendment rights because a foreign bureaucrat sends them an email,” they wrote.

“Under settled principles of US law, American courts will not enforce foreign penal fines or censorship codes.

“If necessary, we will seek appropriate relief in US federal court to confirm these principles.”

They said authorities in the US had been “briefed” on their response to Ofcom’s investigation.

The statement concludes by calling on the Trump administration to invoke all diplomatic and legal levers to protect American businesses from “extraterritorial censorship mandates”.

Ofcom has previously said the Online Safety Act only requires services to take action to protect users based in the UK.

Some American politicians – particularly the Trump administration, its allies and officials – have pushed back against what they regard as overreach in the regulation of US tech firms by the UK and EU.

A perceived impact of the Online Safety Act on free speech has been a particular concern, but other laws have also been the source of disagreement.

On 19 August, US Director of National Intelligence Tulsi Gabbard said the UK had withdrawn its controversial demand for a “backdoor” in an Apple data protection system – saying she worked with the President and Vice President to get the UK to abandon its plan.

Two days later, US Federal Trade Commission chairman Andrew Ferguson warned big tech firms they could be violating US law if they weakened privacy and data security requirements by complying with international laws such as the Online Safety Act.

“Foreign governments seeking to limit free expression or weaken data security in the United States might count on the fact that companies have an incentive to simplify their operations and legal compliance measures by applying uniform policies across jurisdictions,” he said.

If 4chan does successfully fight the fine in the US courts, Ofcom may have other options.

“Enforcing against an offshore provider is tricky,” Emma Drake, partner of online safety and privacy at law firm Bird and Bird, told the BBC.

“Ofcom can instead ask a court to order other services to disrupt a provider’s UK business, such as requiring a service’s removal from search results or blocking of UK payments.

“If Ofcom doesn’t think this will be enough to prevent significant harm, it can even ask that ISPs be ordered to block UK access.”