Conservative firebrand Tucker Carlson, host of Fox News' most popular show, has abruptly left the network, it was announced on Monday, days after the outlet paid a huge settlement to end a defamation case.

Carlson was the best-known name on Fox's presenting roster, hosting a prime-time evening show that won fierce loyalty among the broadcaster's right-wing viewers.

A key figure in Republican politics, he often interviewed Donald Trump, and has been widely criticised for spreading disinformation and his nightly reams of allegedly racist and hate-filled rhetoric.

"Fox News Media and Tucker Carlson have agreed to part ways. We thank him for his service to the network as a host and prior to that as a contributor," the network said in a brief statement, giving no reason for his immediate departure.

The 53-year-old Carlson, who joined Fox in 2009, provided no reaction.

Railing against everything from immigration policies to gun controls, "Tucker Carlson Tonight" pilloried liberal trends in modern America, appealing to viewers' outrage and propelling the show to the heights of cable television.

The US$787.5-million defamation settlement last week meant that neither Fox Corporation chairman Rupert Murdoch nor hosts such as Carlson would have to testify in what was expected to be an explosive trial.

But internal Fox News communications released ahead of the scheduled trial suggested senior figures were prepared to spread falsehoods about the 2020 election for fear of losing viewers to rivals.

In the messages, Carlson also said he couldn't wait until he could "ignore Trump most nights," adding: "I hate him passionately."

Dominion, a voting technology company, had sued Fox for airing false claims that its machines were used to steal the presidential election from Trump.

The Wall Street Journal, which like Fox News is also owned by Murdoch, said on Monday that after the election, Carlson at times disputed that the vote had been rigged.

But Carlson's relationship with Trump appears to still be close, with the former president sitting for a friendly hour-long interview on the show on April 11 after being arraigned on criminal charges in New York.

Despite the criticism Carlson engendered, Fox News had previously backed him as he generated a welcome flood of controversy, media attention, viewers and advertising revenue.

The left-leaning Media Matters group on Monday branded him a "dangerous misinformer" who had "served as the bridge between Fox News and the most extreme parts of the right-wing base."

His "reign illustrates how committed Fox is to lies and extremism," it added.

The network's lawyers argued in a 2020 slander lawsuit that viewers knew to treat material on his show with scepticism, though its tagline was "the sworn enemy of lying, pomposity, smugness and group think." (AFP)