American TV network ABC has announced that it has cancelled Roseanne after the show's star, Roseanne Barr, went on a vitriolic Twitter rampage.

"Roseanne's Twitter statement is abhorrent, repugnant and inconsistent with our values, and we have decided to cancel her show," the network said in a statement released just hours after the comedian's offensive social media rant.

Barr was also dropped by her talent agency, ICM Partners, who announced in a statement that her "disgraceful and unacceptable tweet" was "antithetical to our core values, both as individuals and as an agency. Consequently, we have notified her that we will not represent her."

The cancellation capped a day of online furor over a Twitter rant by Barr in which the actress/comedian spewed false conspiracy theories, went after former first daughter Chelsea Clinton and attacked a former adviser to President Obama with a racist insult. After intense backlash and calls for ABC to fire the comedian and cancel her show, Barr issued a blanket apology and said she was giving up the social media platform. "I apologise. I am now leaving Twitter," she wrote, hours after the tweetstorm began.

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The mea culpa was apparently not enough for comedian Wanda Sykes, a consulting producer for Barr's show. "I will not be returning to @RoseanneOnABC," Sykes wrote shortly after the online tirade ended.

Star Sara Gilbert also chimed in, calling it "incredibly sad and difficult".

Ultimately, it wasn't enough for ABC, which announced the show's cancellation shortly after.

The social-media rant wasn't exactly a surprise performance by Barr, who regularly delights supporters of US President Donald Trump with her retweets of conservative memes and stories. She has trucked in conservative conspiracies before, though she deleted most of her old tweets once it was announced that her show Roseanne was coming back on the air after a 21-year hiatus.

But even by her own standards, the Monday morning tweets were particularly vitriolic. She started by spreading the false rumour that Chelsea Clinton was married to the nephew of billionaire liberal Democratic donor George Soros, who is a lightning rod for false conservative theories.

Clinton responded in her usual M.O. when dealing with social-media weirdness: a clapback cloaked in sweetness. "Good morning Roseanne - my given middle name is Victoria," she wrote. "I imagine George Soros's nephews are lovely people. I'm just not married to one. I am grateful for the important work @OpenSociety (Soros's organisation) does in the world. Have a great day!"

Barr later corrected herself, but added another insult. "CORRECTION: CHELSEA CLINTON IS NOT MARRIED TO A SOROS NEPHEW. HER HUSBAND IS THE SON OF A CORRUPT SENATOR, SO SORRY!" the comedian tweeted. (Fact check: Clinton's father-in-law was convicted of financial fraud, but he was a member of the House, not the Senate.)

Clinton wasn't Barr's only target, though - she had more to say about Soros, whom she accused of wanting to "overthrow ... (US) constitutional republic."

And she called on Americans to "unite against" a CIA mind-control programme, for good measure.

In what might have been her most offensive missive (though it was a tough contest), she appeared to take aim at former Obama adviser Valerie Jarrett, identifying her by her initials: "muslim brotherhood & planet of the apes had a baby=vj," she wrote, though she later claimed it was "a joke".

After more intense criticism directed to her and ABC, she issued a fuller apology. "I apologise to Valerie Jarrett and to all Americans," she wrote.

"I am truly sorry for making a bad joke about her politics and her looks. I should have known better. Forgive me - my joke was in bad taste."

Barr has tweeted in the past about "Pedogate," the term for conspiracy theories that accuse people (usually Trump opponents) of being part of a secret ring of paedophiles, and Pizzagate (the most famous of those theories).

She also tweeted last year about Seth Rich, the Democratic National Committee staffer who was shot in Washington, echoing a conservative theory that his death was covered up by the Clintons. "THE FIX IS IN!" Barr tweeted at the time.

Roseanne, in which Barr plays the Trump-loving matriarch of a blue-collar family, had just been renewed for a second season.

As for whether Barr will actually stick to her promise to stay off Twitter? It's unclear: She last quit the platform ... 10 days ago.