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

The officials, who spoke to reporters travelling with Mr Trump to Washington from Florida aboard Air Force One on Wednesday (local time), gave no further details.

Mr Trump had earlier threatened to regulate or shut down social media companies, after Twitter attached fact checks to some of the his tweets.

Without offering evidence, Mr Trump again accused such platforms of bias against conservatives, tweeting: “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.”

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

Twitter has now shown that everything we have been saying about them (and their other compatriots) is correct. Big action to follow!

— Donald J. Trump (@realDonaldTrump) May 27, 2020

Mr Trump’s threat to shut down platforms such as Twitter and Facebook was his strongest yet within a broader conservative backlash against Big Tech. Shares in both companies fell.

On Tuesday, Twitter for the first time attached fact-check labels on Trump tweets after he made unsubstantiated claims about mail-in voting.

In a pair of early morning posts on Wednesday, the Republican president again blasted mail-in ballots. Mr Trump falsely claims that mail-in ballots lead to fraud and ineligible voters getting ballots.

Twitter and Facebook declined to comment on Mr Trump’s tweets. Asked during Twitter’s annual meeting on Wednesday why the company decided to affix the label to Mr Trump’s mail-in ballot tweets, general counsel Sean Edgett said decisions about handling misinformation were 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.

Mr Trump’s campaign manager, Brad Parscale, said Twitter’s “clear political bias” had led the campaign to pull “all our advertising from Twitter months ago”.

Twitter has banned all political advertising since November 2019.

In recent years, the company 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.

The Internet Association, which includes Twitter and Facebook among its members, said online platforms had no political bias and they offered “more people a chance to be heard than at any point in history”.

It was not immediately clear whether Mr Trump has the authority to shut down the companies. The American Civil Liberties Union said the First Amendment of the US Constitution limited any action Mr Trump could take to regulate social media platforms.

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

-with agencies