Social media platforms need to be “more accountable” in how they curate and police content on their platforms, or face regulation, the head of Ofcom has said.

The threat comes as Facebook stares down a barrage of criticism in the US for its refusal to bar the US conspiracy theorist website InfoWars from its platform, even as it mounts an international ad campaign claiming that “fake news is not our friend”.

Writing in the Times on Friday, Ofcom’s chief executive, Sharon White, said: “The argument for independent regulatory oversight of [large online players] has never been stronger.

“In practice, this would place much greater scrutiny on how effectively the online platforms respond to harmful content to protect consumers, with powers for a regulator to enforce standards, and act if these are not met.”

White highlighted Ofcom research that demonstrated how little trust user have in what they read on social media. “Only 39% consider social media to be a trustworthy news source, compared with 63% for newspapers, and 70% for TV,” she wrote.

“Many people admit they simply don’t have the time or inclination to think critically when engaging with news, which has important implications for our democracy.”

White joins a growing number of British power brokers arguing that social media are under-regulated. The government has frequently suggested as much, with Matt Hancock, then digital, culture, media and sport secretary, telling Facebook in April: “Social media companies are not above the law and will not be allowed to shirk their responsibilities to our citizens.

“We will do what is needed to ensure that people’s data is protected and don’t rule anything out. That includes further regulation in the future.”

Facebook has been on the offensive recently, buying newspaper and billboard advertising space in the UK and US to say“data misuse”, “fake news” and “clickbait” are “not our friend”.

But the company’s strategy hit a roadblock on Thursday, when a meeting with reporters ostensibly aimed at demonstrating new tools for fighting fake news instead became dominated by the question of what Facebook should do about one of the most prominent purveyors of hoaxes on its platform: InfoWars.

Asked by a CNN reporter how Facebook could seriously argue that it cared about cracking down on misinformation while allowing InfoWars a page with almost a million followers,its head of news feed, John Hegeman, responded that the company did not “take down false news”.

Hegeman added that “just being false” did not violate the community standards. “We created Facebook to be a place where different people can have a voice. And different publishers have very different points of view,” he continued.

Others present noted that InfoWars has published stories that claimed that the Democrats were planning a civil war for 4 July and that the school shooting in Parkland, Florida, was a “false flag”, and questioned whether those counted as different points of view.

The exchange rapidly boiled over into a public dispute, with Facebook’s official Twitter account engaging in back-and-forth debate with journalists on the issue. “We just don’t think banning pages for sharing conspiracy theories or false news is the right way to go,” it told the New York Times’ Kevin Roose. He responded by quoting Facebook’s own words this year: “In February, you called crisis actor hoaxes ‘abhorrent’ and said you would remove them. But now you’re saying they shouldn’t be removed, just demoted?”

InfoWars also joined in the row, leading its site with the headline: “CNN lobbies Facebook to shut down InfoWars”,and accusing the news channel of “bullying, lies and intimidation”.