Two of the most recognizable names in cable news —  on opposite ends of the political spectrum — were both suddenly and unceremoniously removed from their anchors’ chairs Monday by their respective networks in what amounts to a seismic shakeup in the television news landscape.  

On the right, one of Fox News’ most popular primetime hosts, Tucker Carlson, is leaving the network effective immediately, Fox News said in a terse statement. His last show on the network was Friday, April 21. On the left, CNN announced that it was terminating longtime host Don Lemon from its lineup, also effective immediately.

“Fox News media and Tucker Carlson have agreed to part ways,” a statement from Fox News said. “We thank him for his service to the network as a host and prior to that as a contributor.”

The statement from CNN was equally abrupt. “CNN and Don have parted ways,” the network said. “Don will forever be a part of the CNN family, and we thank him for his contributions over the past 17 years. We wish him well and will be cheering him on in his future endeavors.”

In a post on Twitter, Mr. Lemon said his departure from the network was far from voluntary and even less amicable. “I was informed this morning by my agent that I have been terminated by CNN,” Mr. Lemon, who appeared on air Monday morning as normal. “I am stunned. After 17 years at CNN I would have thought that someone in management would have had the decency to tell me directly. At no time was I ever given any indication that I would not be able to continue to do the work I have loved at the network.”

CNN later said Mr. Lemon’s description of the events were incorrect. “He was offered an opportunity to meet with management but instead released a statement on Twitter,” the company said in its own tweet.

CNN said the show Mr. Lemon was only recently moved to, “CNN This Morning,” which he co-hosted with Poppy Harlow and Kaitlan Collins, would continue as usual. “‘CNN This Morning’ has been on the air for nearly six months, and we are committed to its success,” CNN said in its statement.

Fox News said that beginning Monday night, the 8 p.m. slot previously filled by Mr. Carlson’s eponymous show, “Tucker Carlson Tonight,” will be filled by a show titled “Fox News Tonight” helmed by what was described as a rotating collection of Fox News personalities until a new host can be named.

No reason was given immediately for Mr. Carlson’s departure, but the abruptness of the announcement and the fact that he will not be airing a final show suggests that the separation was not something that had been planned. The network was still airing promotional spots for his show Monday morning prior to the announcement, and he signed off on his Friday show by saying, “We’ll be back on Monday.”

Multiple media outlets reported Monday that Mr. Carlson was “totally surprised” by his termination. The parent company of Fox News, Fox Corporation, plunged on the news, losing nearly five percent of its value before recovering slightly in afternoon trading. The stock was on track to close down by more than three percent for the day.

The separation comes just days after Fox agreed to settle a defamation case against it by Dominion Voting Systems for more than $787 million. In its lawsuit, Dominion said Fox and its primetime personalities — including Mr. Carlson — defamed the company by repeating claims from President Trump and his lawyers that the company somehow rigged the 2020 election against him. Mr. Carlson was widely expected to testify had the case gone to trial.

Mr. Carlson’s show was consistently one of the highest-rated offerings on the network — indeed, in all of cable news — with more than 3 million nightly viewers. He was one of the network’s most vociferous supporters of Mr. Trump.

CNN also was mum about the reasons for Mr. Lemon’s departure, but the famously outspoken liberal came under fire several weeks ago for remarks about GOP presidential contender Nikki Haley, who he said was not “in her prime.” Many both in the network and outside of it called the remarks sexist. Not long afterward, articles began appearing in the popular press — including one in Variety — detailing what was alleged to be a long history of misogyny and diva-like antics by the anchor.