A federal appeals court ruled on Friday that the Biden administration most likely overstepped the First Amendment by urging the major social media platforms to remove misleading or false content about the Covid-19 pandemic, partly upholding a lower court’s preliminary injunction in a victory for conservatives.

The ruling, by a three-judge panel of the U.S. Court of Appeals for the Fifth Circuit in New Orleans, was another twist in a First Amendment case that has challenged the government’s ability to combat false and misleading narratives about the pandemic, voting rights and other issues that spread on social media.

The judges wrote that the White House and the Office of the Surgeon General had “coerced the platforms to make their moderation decisions by way of intimidating messages and threats of adverse consequences” and “significantly encouraged the platforms’ decisions by commandeering their decision-making processes.”

The appellate court also found that the Federal Bureau of Investigation had used coercion in its interactions with the companies, which took down 50 percent of the material online that the bureau’s agents flagged as troublesome.

“Given the record before us, we cannot say that the F.B.I.’s messages were plainly threatening in tone or manner,” the judges wrote. Nevertheless, “we do find the F.B.I.’s requests came with the backing of clear authority over the platforms.”

The court limited the scope of a preliminary injunction, which prohibited officials from numerous agencies from having practically any contact with the social media companies. Instead, the court narrowed the impact to the White House, the Surgeon General’s Office, the F.B.I., and the Centers for Disease Control and Prevention.

The White House defended its interactions with social media companies and said the Department of Justice was reviewing the ruling and would consider options for responding.

“This administration has promoted responsible actions to protect public health, safety and security when confronted by challenges like a deadly pandemic and foreign attacks on our elections,” the White House said in a statement. “Our consistent view remains that social media platforms have a critical responsibility to take account of the effects their platforms are having on the American people but make independent choices about the information they present.”

Jenin Younes, a lawyer with the New Civil Liberties Alliance, an organization representing individual plaintiffs in the case, called the decision on Friday “a major and unprecedented victory.”

“This might be the most significant First Amendment case in the internet age and is a crucial outcome for flourishing of free speech in an era when social media has become the modern public square,” she said.

The attorneys general of Missouri and Louisiana, both Republicans, argued in a lawsuit filed last year that government agencies and officials — including some working in the administration of President Donald J. Trump — had abused their authority by coercing companies like Facebook, Twitter (now called X) and YouTube to silence critics.

Judge Terry A. Doughty of U.S. District Court for the Western District of Louisiana agreed, ordering a preliminary injunction against the government. In a ruling issued on July 4, he said the accusations in the lawsuit arguably involved “the most massive attack against free speech in United States’ history.”

On the question of First Amendment protections, the three-judge panel of the appellate court, which last month temporarily froze Judge Doughty’s injunction, largely agreed.

The debate over how far companies can go to limit content online — known as moderation — has become increasingly vehement and polarizing. On one side, government officials have argued that they have a duty to protect public health and national security from false or misleading information. Republicans and others, however, have accused the social media giants of colluding with government officials in violation of First Amendment protections of free speech.

They have focused their anger at the Biden administration even though previous administrations regularly had contacts with social media companies and some of the instances cited in the lawsuit occurred during the Trump administration.

Yoel Roth, the former head of trust and safety at Twitter, noted recently that Mr. Trump’s White House had asked the company in 2019 to remove a tweet in which the model Chrissy Teigen called the president several expletives. (The company did not, after what Mr. Roth called Kafkaesque internal deliberations.)

Government officials have long argued that they do not have the authority to order posts or entire accounts removed from the platforms, which private companies control. They have worked with the tech giants, however, to take action against illegal or harmful material, especially in cases involving child sexual abuse, human trafficking and other criminal activity.

That has also included regular meetings to share information on the Islamic State and other terrorist groups. Many of the cases cited in the legal challenge involved the Covid pandemic, when government officials feared that misinformation and disinformation about vaccines and other treatments hampered efforts to control the spread of the coronavirus, which has killed more than 1.1 million Americans.

Although Republicans have led the charge, the argument that the government has overstepped its constitutional authority to police content online has raised concerns across the political spectrum. It has drawn support from others who complain about the role of social media giants in moderating content on their platforms, including hate speech and misinformation and disinformation.

Robert F. Kennedy Jr., the challenger for the Democratic presidential nomination and promoter of a number of conspiracy theories, filed a similar lawsuit, which was consolidated into the Missouri case. He argued that government officials had forced platforms to remove accounts, including his.

“Never in the history of this country have federal officials worked so blatantly in collusion with industry to silence the voices that question government agendas,” Mr. Kennedy, who leads the Children’s Health Defense, an anti-vaccine organization, said in a statement ahead of the appellate hearing in New Orleans.

Other groups weighed in on the side of the government, arguing that the lower court’s injunction would interfere with the free speech of researchers or others who called the attention of the government and the companies to harmful material on their platforms.

The Lawyers’ Committee for Civil Rights Under the Law, an advocacy group, argued in an amicus brief to the appellate court that the judge’s injunction was so broadly and vaguely written that it would “chill critical information sharing” among researchers, companies and government officials before the 2024 presidential election.

“In the United States, malicious actors have repeatedly deployed election falsehoods to confuse and deter voters,” the group argued. “This threat persists as a new presidential election approaches.”