Jeff Zucker resigned on Wednesday as the president of CNN and the chairman of WarnerMedia’s news and sports division, writing in a memo that he had failed to disclose to the company a romantic relationship with another senior executive at CNN.

Mr. Zucker, 56, is among the most powerful leaders in the American media and television industries. The abrupt end of his nine-year tenure immediately throws into flux the direction of CNN and its parent company, WarnerMedia, which is expected to be acquired later this year by Discovery Inc. in one of the nation’s largest media mergers.

In a memo to colleagues that was obtained by The New York Times, Mr. Zucker wrote that his relationship came up during a network investigation into the conduct of Chris Cuomo, the CNN anchor who was fired in December over his involvement in the political affairs of his brother, former Governor Andrew M. Cuomo of New York.

“As part of the investigation into Chris Cuomo’s tenure at CNN, I was asked about a consensual relationship with my closest colleague, someone I have worked with for more than 20 years,” Mr. Zucker wrote. “I acknowledged the relationship evolved in recent years. I was required to disclose it when it began but I didn’t. I was wrong.”