Donald Trump's practice of blocking Twitter users who criticse or mock him is unconstitutional, a US federal appeals court ruled on Tuesday.
Mr Trump, a prolific tweeter, has prohibited some members of the public from reading his posts after criticising the US president.
They include Rebecca Buckwalter, who was blocked by Mr Trump after replying to one of his posts about winning the White House with the tweet: “To be fair you didn’t win the WH: Russia won it for you”.
The three-judge panel agreed with a lower court's ruling last year that the president was using "viewpoint discrimination" in violation of the constitutional rights of people with opposing views.
The Second Circuit Appeals Court said that Mr Trump had effectively created a public forum for official White House business.
Very good numbers on the economy. Much potential for growth. Trade deals being negotiated or being set up for negotiation. We have been treated very unfairly (to put it mildly) by other countries for many years, but that is changing!— Donald J. Trump (@realDonaldTrump) July 9, 2019
It comes after a group of Twitter users and the Knight First Amendment Institute at Columbia University filed a lawsuit against the president, accusing him of improperly blocking comments from his political opponents.
The plaintiffs, which included a University of Maryland professor, a Texas police officer and a New York comic, said they were blocked from the @realDonaldTrump account after posting tweets critical of his policies.
Mr Trump's legal team had argued that he is not acting in his official capacity when he blocks users, but the court disagreed.
"The president and multiple members of his administration have described his use of the account as official," the appeals court ruling said.
"We conclude that the evidence of the official nature of the account is overwhelming. We also conclude that once the president has chosen a platform and opened up its interactive space to millions of users and participants, he may not selectively exclude those whose views he disagrees with."
The Justice Department, which represented the president, has the option to appeal the decision to the US Supreme Court but has not yet commented on its next step.