The California Supreme Court unanimously agreed on Wednesday to hear a lower-court case that some Internet companies say has the potential to chill free speech online.

The case was brought by San Francisco lawyer Dawn Hassell, who said a former client named Ava Bird posted defamatory reviews of her on Yelp. She sued Bird, who did not appear in court, and a San Francisco judge ruled that the reviews were indeed defamatory. The judge ordered Yelp to remove the reviews in a decision that was upheld by a second judge and a state appeals court.

But Yelp, as well as a who’s who of Internet companies, said the case would create a dangerous precedent, opening a pathway for people or companies to gut online content they find distasteful. It has not taken down the reviews, pending the appeal.

Aaron Schur, Yelp’s senior director of litigation, said in a statement that the San Francisco reviews site is happy the high court will hear the case: “We look forward to making our full arguments to the Court and explaining how the lower court's decision is ripe for abuse, contradicts long-standing legal principles, and restricts the ability of websites to provide a balanced spectrum of views online.”

Monique Olivier, a San Francisco lawyer who represented Hassell, said the lawsuit is being misrepresented as a free-speech issue.

“That simply is not the case,” she said. “The speech at issue here is not protected speech but speech adjudicated to be defamatory. There are decades of jurisprudence finding that defamatory speech is not protected by the First Amendment.”

Eric Goldman, co-director of the High Tech Law Institute at Santa Clara University, had a different take.

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“You could imagine the floodgates opening up with businesses trying to do exactly what Hassell did to get rid of unwanted reviews on any user-generated content site anywhere on the Web,” he said.

Basically the case creates a playbook that other businesses could use to remove unwanted content, Goldman said. “Step 1: Business sues review author. Step 2: Review author doesn’t appear in court or agrees to drop out of the case. Step 3: Business asks the court for an injunction requiring removal of the review.”

Since Bird, the reviewer, did not appear in court, Goldman said that meant the original court only heard one side of the case — that of Hassell — and therefore was more likely to rule for that side by default, finding that the material was defamatory and targeting it for removal. But Olivier said that the decision was based on evidence and testimony for Hassell’s side, and was not a default judgment.

Yelp was not a party to the original lawsuit and so did not get a day in court to make its case.

“The California appeals court said Yelp had no standing to protest an injunction against it,” Goldman said. “That contradicts basic due process that we learned in kindergarten. Yelp was ordered to do something without ever having a chance to tell the court its side of the story.”

The appeals court said that the case did not violate the 1996 Communications Decency Act, a federal law that shields Internet companies for liability for online content, because Yelp was not declared liable, but simply ordered to remove the disputed content.

The state Supreme Court is likely to take at least a year to issue a ruling.

Many Internet companies are watching the case with keen interest.

The lower-court ruling could be used to “silence a vast quantity of protected and important speech,” Facebook, Twitter and Microsoft wrote in a letter to the California Supreme Court last month.

Carolyn Said is a San Francisco Chronicle staff writer. Email: csaid@sfchronicle.com Twitter: @csaid