Billionaire Bill Ackman says he is suing Business Insider for defamation

Billionaire Bill Ackman has taken the gloves off in his latest public battle, this time vowing to sue Business Insider for defamation after the outlet published two stories accusing his wife, Neri Oxman, of plagiarism.

In reaction to a report that BI is standing by its reporting, Ackman said on X that BI and owner Axel Springer "have tripled down on their false claims and defamation," and said he and his wife would "respond in a formal complaint which will take a few weeks to prepare." He noted, "By complaint I mean lawsuit, to be clear."

Earlier in the day, the Pershing Square Capital Management founder and CEO posted that BI "is toast," and said it would look something like the "unleash hell" scene from the 2000 film "Gladiator."

Earlier this month, BI reported that Oxman plagiarized in her doctoral dissertation. Oxman, a former MIT professor, acknowledged omitting quotation marks and apologized in a post on X, and added that she will check the citations and "request that MIT make any necessary corrections."

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BI then published a follow-up piece accusing Oxman of plagiarizing portions of her dissertation and other papers, including passages from Wikipedia and other scholarly and technical writings.

Ackman pushed back, questioning the accuracy and fairness of the reporting.

Then on Saturday, BI CEO Barbara Peng posted a note on the outlet's website defending its reporting on Oxman. When reached Monday for reaction to Ackman's lawsuit threat, a BI spokesperson told FOX Business they had no further comment beyond Peng's letter.

BI's reports regarding Ackman's wife came after the resignation of Harvard President Claudine Gay, who stepped down on Jan. 2 amid mounting claims that she plagiarized several times in her scholarly work.

Ackman had led calls for Gay, University of Pennsylvania President Liz Magill and MIT President Sally Kornbluth to "resign in disgrace" after their controversial testimonies at a House committee hearing in early December on the rise of antisemitism on college campuses, when all three refused to say that calling for the genocide of Jews on their respective campuses breached their rules and amounted to harassment.

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Magill resigned within days, but Harvard stood by Gay for weeks until she ultimately tendered her resignation as the plagiarism claims continued to climb. She remains a member of Harvard's faculty.

FOX Business' Eric Revell contributed to this report.