SAN FRANCISCO — The University of California, Berkeley, on Wednesday canceled a scheduled speech by the conservative author Ann Coulter, in the latest blow to the institution’s legacy and reputation as a promoter and bastion of free speech.

University administrators said in a statement that they could not let Ms. Coulter speak because of active security threats. In a letter to the Berkeley College Republicans, which was sponsoring the speech, two vice chancellors said the university had been “unable to find a safe and suitable venue for your planned April 27 event featuring Ann Coulter.”

The letter, written by Scott Biddy, the vice chancellor, and Stephen Sutton, the vice chancellor for student affairs, said it was “not possible to assure that the event could be held successfully — or that the safety of Ms. Coulter, the event sponsors, audience and bystanders could be adequately protected.”

After the cancellation was announced on Wednesday, Ms. Coulter said in an email that she would “definitely” speak at Berkeley next week.