A federal judge in Louisiana on Tuesday restricted parts of the Biden administration from communicating with social media platforms about broad swaths of content online, a ruling that could curtail efforts to combat false and misleading narratives about the coronavirus pandemic and other issues.

The ruling, which could have significant First Amendment implications, is a major development in a fierce legal fight over the boundaries and limits of speech online.

Republicans have often accused the government of inappropriately working with social media sites like Facebook, Twitter and YouTube to censor critics and say the platforms disproportionately take down right-leaning content. Democrats say the platforms have failed to adequately police misinformation and hateful speech, leading to dangerous outcomes, including violence.

In the ruling, Judge Terry A. Doughty of the U.S. District Court for the Western District of Louisiana said that parts of the government, including the Department of Health and Human Services and the Federal Bureau of Investigation, could not talk to social media companies for “the purpose of urging, encouraging, pressuring, or inducing in any manner the removal, deletion, suppression, or reduction of content containing protected free speech.”

Judge Doughty said in granting a preliminary injunction that the agencies could not flag specific posts to the social media platforms or requests reports about their efforts to take down content. The ruling said that the government could still notify the platforms about posts detailing crimes, national security threats or foreign attempts to influence elections.

Courts are increasingly being forced to weigh in on such issues — with the potential to upend decades of legal norms that have governed speech online.

The Republican attorneys general of Texas and Florida are defending first-of-their-kind state laws that bar internet platforms from taking down certain political content, and legal experts believe those cases may eventually reach the Supreme Court. The high court earlier this year declined to limit a law that allows the platforms to escape legal liability for content that users post to the sites.

The ruling on Tuesday, in a lawsuit brought by the attorneys general of Louisiana and Missouri, is likely to be appealed by the Biden administration, but its impact could be sweeping, forcing government officials, including law enforcement agencies, to refrain from notifying the platforms of troublesome content.

Government officials have argued they do not have the authority to order posts or entire accounts removed, but they have long cooperated with Big Tech 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.

A White House spokesperson said the Justice Department was reviewing the ruling and evaluating its possible next steps.

“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 spokesperson 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.”

Meta, which owns Facebook and Instagram, declined to comment. Twitter did not have a comment, and Google did not respond to a request for comment.

Senator Eric Schmitt, Republican of Missouri and previously the state’s attorney general, said on Twitter that the ruling was a “win for the First Amendment on this Independence Day.”

The issue of the government’s influence over social media has become increasingly partisan.

The Republican majority in the House has taken up the cause, smothering universities and think tanks that have studied the issue with onerous requests for information and subpoenas.

Since acquiring Twitter last year, Elon Musk has pushed a similar argument, releasing internal company documents to chosen journalists suggesting what they claimed was collusion between company and government officials. Though that remains far from proven, some of the documents Mr. Musk disclosed ended up in the lawsuit’s arguments.

The defendants, the social media companies and experts who study disinformation have argued that there is no evidence of a systematic effort by the government to censor individuals in violation of the First Amendment.

At the same time, emails and text messages made public in the case that Judge Doughty ruled on have shown instances where officials complained to social media executives when influential users spread disinformation, especially involving the coronavirus pandemic.

The ruling came in a lawsuit filed last year by the Missouri and Louisiana attorneys general, both Republicans, and four other individual plaintiffs: Jayanta Bhattacharya and Martin Kulldorff, epidemiologists who questioned the government’s handling of the pandemic; Aaron Kheriaty, a professor dismissed by the University of California, Irvine, for refusing to have a coronavirus vaccination; Jill Hines, a director of Health Freedom Louisiana, an organization that has been accused of disinformation; and Jim Hoft, founder of Gateway Pundit, a right-wing news site.

Although the lawsuit named as defendants President Biden and dozens of officials in 11 government agencies, some of the instances cited took place during the Trump administration.

Judge Doughty, who was appointed to the federal court by President Donald J. Trump in 2017, has made the court a sympathetic venue for conservative cases, having previously blocked the Biden administration’s national vaccination mandate for health care workers and overturned its ban on new federal leases for oil and gas drilling.

He allowed the plaintiffs extensive discovery and depositions from prominent officials like Anthony S. Fauci, then the nation’s top infectious disease expert, who told the plaintiffs’ lawyers that he was not involved in any discussions to censor content online.

Judge Doughty signaled his skepticism of that argument in March when he denied a motion to dismiss the case.

Some experts in First Amendment law and misinformation criticized the Tuesday ruling.

“It can’t be that the government violates the First Amendment simply by engaging with the platforms about their content-moderation decisions and policies,” said Jameel Jaffer, the executive director of the Knight First Amendment Institute at Columbia University. “If that’s what the court is saying here, it’s a pretty radical proposition that isn’t supported by the case law.”

Mr. Jaffer added that the government has to balance between calling out false speech without stepping into informal coercion that veers toward censorship. “Unfortunately Judge Doughty’s order doesn’t reflect a serious effort to reconcile the competing principles,” he said.

Judge Doughty’s ruling said the injunction would remain in place while proceedings in the lawsuit continued unless he or a higher court ruled differently.

Emma Goldberg contributed reporting.