Trump to sign an executive order aimed at social media firms on Thursday

US President Donald Trump will sign an executive order on social media companies on Thursday, White House officials said after Trump threatened to shut down the platform he accused of stifling conservative voices.

The officials, who spoke to reporters traveling with Trump to Washington from Florida aboard Air Force One on Wednesday, gave no further details.

Before leaving for Florida earlier in the day to observe a space launch that was postponed because of bad weather, Trump again accused Twitter Inc and other social media of bias without offering evidence. It was unclear how Trump could follow through on the threat of shutting down social media companies.

The American Civil Liberties Union said the First Amendment of the US Constitution limits any action Trump could take to regulate such platforms.

Twitter declined to comment on news of Trump's plans. Facebook and Google did not immediately comment.

Separately, a three-judge panel of the US Court of Appeals in Washington on Wednesday upheld the dismissal of a suit brought by a conservative group and right-wing YouTube personality against Google, Facebook, Twitter and Apple alleging they conspired to suppress conservative political views.

Trump's latest dispute with social media emerged after Twitter on Tuesday for the first time attached a warning to some of his tweets prompting readers to fact check the president's claims.

In the tweets tagged by Twitter, Trump made unsubstantiated claims about mail-in voting. Trump falsely claims that mail-in ballots lead to vote fraud and ineligible voters getting ballots.

"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," Trump said in a pair of additional posts on Twitter on Wednesday.

The president, a heavy user of Twitter with more than 80 million followers, added: "Clean up your act, NOW!!!!"

Strongest threat yet

Trump's threat to shut down platforms like Twitter and Facebook is his strongest yet within a broader conservative backlash against Big Tech. The shares of both companies fell on Wednesday.

Last year the White House circulated drafts of a proposed executive order about anti-conservative bias which never gained traction.

The Internet Association, which includes Twitter and Facebook among its members, said online platforms do not have a political bias and they offer "more people a chance to be heard than at any point in history."

Asked during Twitter's annual meeting on Wednesday why the company decided to affix the label to Trump's mail-in ballot tweets, General Counsel Sean Edgett said decisions about handling misinformation are made as a group.

"We have a group and committee of folks who take a look at these things and make decisions on what's getting a lot of visibility and traction...," he said.

In recent years Twitter has tightened its policies amid criticism that its hands-off approach allowed fake accounts and misinformation to thrive.

Tech companies have been accused of anti-competitive practices and violating user privacy. Apple, Google, Facebook and Amazon face antitrust probes by federal and state authorities and a US congressional panel.

Republican and Democratic lawmakers, along with the US Justice Department, have been considering changes to Section 230 of the Communications Decency Act, a federal law largely exempting online platforms from legal liability for the material their users' post. Such changes could expose tech companies to more lawsuits.

Republican Senator Josh Hawley, a frequent critic of Big Tech companies, sent a letter to Twitter Chief Executive Jack Dorsey on Wednesday asking why the company should continue to receive legal immunity after "choosing to editorialize on President Trump's tweets."

Twitter's rival Facebook left Trump's post on mail-in ballots on Tuesday untouched.