Billionaire Ronald Lauder, a powerful financial backer of the University of Pennsylvania, is threatening to cut off donations if the school doesn't do more to fight antisemitism, CNN has learned.

The threat from Lauder, one of the heirs to the Estee Lauder cosmetics company, marks the latest fallout from donors and alumni alarmed by a Palestinian literary festival that was held on campus last month prior to the Hamas terror attacks on Israel.

Even before the Palestine Writes Literature Festival started, UPenn leaders acknowledged it would include speakers with a history of making antisemitic remarks.

"The conference has put a deep stain on Penn's reputation that will take a long time to repair," Lauder wrote to UPenn President Liz Magill on Monday in a letter obtained by CNN.

"You are forcing me to reexamine my financial support absent satisfactory measures to address antisemitism at the university," Lauder wrote.

Lauder said he had two people taking photos at the Palestine Writes festival and two more who listened to the speakers, who were "antisemitic and viscerally anti-Israel."

UPenn did not respond to a request for comment on the Lauder letter.

Magill, UPenn's president, conceded over the weekend that the response to the Palestine Writes Literature Festival was inadequate.

"While we did communicate, we should have moved faster to share our position strongly and more broadly with the Penn community," Magill said in a statement Sunday.

The UPenn leader said she knows how "painful the presence of these speakers" on campus was for the Jewish community, especially during the holiest time of the Jewish year.

"The University did not, and emphatically does not, endorse these speakers or their views," Magill said.

This is a breaking news story. It will be updated.