CNN, MS NOW and Politico have said they have notified Donald Trump's administration that they are filing a lawsuit over being denied White House access.
Reporters from the three major news outlets were blocked from the White House on Saturday, a day after Mr Trump said he was banning them because of reporting what he called "fake news".
"Without notice or process, the White House revoked our journalists' credentials because it objected to our reporting," the publications said in a joint statement on X on Monday.
"Left unchallenged, this threatens press freedom and the public's right to independent journalism free from government interference."
All three news outlets had hinted at legal action, saying they would vigorously defend their First Amendment rights.
After the lawsuit was announced, Mr Trump defended the ban in a Truth Social post, calling the news media a security threat.
"The White House is not instituting an assault on the Free Press, something which I cherish. It is instituting an assault on the FAKE NEWS," he said on Monday.
"It is corrupt, purposeful, pervasive, fully coordinated, and totally out of control. It is a threat to our National Security, and must be stopped, NOW!"
More from Sky News:
South Korean soldiers injured in landmine border explosion
Converse apologises and removes ad over backlash
On Saturday, White House reporters from the two cable networks - Betsy Klein of CNN and Akayla Gardner of MS NOW - went live to report they had been turned away, and had their press badges deactivated when arriving for their jobs at the White House.
Shortly afterwards, Politico said its White House reporter Cheyenne Haslett was also denied access and had her badge taken away.
In his second term, Mr Trump and his Republican administration have punished certain media outlets, both in the courtroom and through regulatory action.
The US president has criticised individual reporters in person and on social media. He has insulted them in media briefings and also in his recent speech at the White House Correspondents' Association (WHCA) dinner.
The WHCA's president, Jacqui Heinrich, called on the Trump administration to immediately restore the three outlets' access.
"The implications extend beyond these organisations: a standard used to exclude one news outlet because of its coverage could be applied to any outlet in the future," Ms Heinrich said.
"The American people, through a free and independent press, must be able to scrutinise those elected to power, regardless of whether government officials view it favourably."
(c) Sky News 2026: CNN, MS NOW and Politico 'to sue' over White House ban
Southport killer Axel Rudakubana accused of throwing boiling water mixed with sugar at prison officer
Flood and landslide warnings as typhoon approaches Tokyo
Forger made £2.7m from largest fake vinyl haul ever found by police in the UK
