Donald Trump is to sign an executive order on social media companies, White House officials have said.
The US president accused the technology firms of trying to stifle right-wing opinions on Wednesday after Twitter tagged one of his tweets about unsubstantiated claims of postal vote fraud with a fact checker warning.
It also came after a three-judge panel of the US Court of Appeals in Washington on Wednesday upheld the dismissal of a lawsuit by a conservative group and right-wing YouTube personality against Google, Facebook, Twitter and Apple.
Laura Loomer and Freedom Watch accused them of conspiring to suppress conservative political views.
But the courts decided they do not have any viable claims that the companies violated their First Amendment free speech rights.
The president said: “Republicans feel that social media platforms totally silence conservatives voices. We will strongly regulate, or close them down, before we can ever allow this to happen.”
Mr Trump, a heavy user of Twitter with more than 80 million followers, added: “Clean up your act, NOW!!!!”
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The White House gave no further details on what the order, which will be signed on Thursday, would involve.
Shares of both Twitter and Facebook fell on Wednesday.
It was unclear how Mr Trump could follow through on the threat of shutting down privately-owned companies, including Twitter.
Twitter chief executive Jack Dorsey said Mr Trump’s tweets about California’s postal voting plans “may mislead people into thinking they don’t need to register to get a ballot”.
On its main account, Twitter said Mr Trump’s tweets were labelled as part of efforts to enforce the company’s “civic integrity policy.”
The policy document on Twitter’s website says people may not use its services for manipulating or interfering in elections or other civic processes.
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In an interview with Fox News on Wednesday, Facebook chief executive Mark Zuckerberg, who did not tag Mr Trump’s mail-out ballots post on the website, said censoring a platform would not be the “right reflex” for a government worried about censorship.
The American Civil Liberties Union said the First Amendment of the US Constitution limits any action Mr Trump could take.
Facebook and Alphabet’s Google declined to comment. Apple did not respond to a request for comment.
The move is not the first time the president has been at odds with social media.
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Last year, his administration drafted an executive order, titled “Protecting Americans from Online Censorship”.
It aimed to restrict the ability of platforms to remove content and give the Federal Communications Commission a far larger role in policing content.
The Federal Trade Commission would then be empowered to sue companies over their regulation and editorial policies.
Agency officials argued it was an unconstitutional order. The order ended up not being progressed further.