The left-leaning Guardian, which has 10.7 million followers on X, becomes the first large UK media company to retreat from the platform that Elon Musk purchased in 2022.
Critics say Musk's hands-off approach has allowed lies and hate speech to spread on the platform formerly known as Twitter.
"We think that the benefits of being on X are now outweighed by the negatives and that resources could be better used promoting our journalism elsewhere," the Guardian said in an editorial published on its website.
"This is something we have been considering for a while given the often disturbing content promoted or found on the platform, including far-right conspiracy theories and racism."
In response, Musk posted on X and said of the Guardian: "They are irrelevant."
Musk, who supported Donald Trump ahead of his US election victory, has said he is defending freedom of speech.
Trump on Tuesday named Musk to a role aimed at creating a more efficient government.
X and other platforms came under the spotlight in Britain when far-right and racist violence broke out after online posts falsely claimed that an attack in the northern English town of Southport, where three young girls were killed, was the work of an Islamist migrant.
Reuters was first to report in October that a British police force had quit posting on X, with several more reviewing their involvement.
In recent months, some British charities, health and educational establishments have said they will no longer post to X.
Britain's government continues to post on X but does not use it for paid communications. It does, however, advertise on Meta's Instagram and Facebook, a government source told Reuters last month.
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