The chairman of Fox News demanded an apology from Donald Trump after he resumed attacks on popular TV host Megyn Kelly, dramatically re-igniting a feud between the broadcaster and the Republican frontrunner.
Trump used his Twitter account late on Monday to slam Kelly as she returned from vacation to host her show.
It's a revival of his brutal criticism of her after she asked pointed questions of the bombastic billionaire during an August 6 debate featuring Republican presidential hopefuls.
Trump retweeted a comment from a user late Monday calling Kelly a "bimbo," and said her show "was much better without Megyn Kelly. Her replacement while she was out on vacation was much better!"
Kelly likely had a "terrible vacation" because "she is really off her game," Trump added.
The comments drew strong reaction from Fox chairman Roger Ailes, a former Republican adviser and strategist who still holds tremendous sway within the party.
"Donald Trump's surprise and unprovoked attack on Megyn Kelly during her show last night is as unacceptable as it is disturbing," Ailes said in a statement read on the air by a Fox News anchor on Tuesday.
Kelly "represents the very best of American journalism and all of us at Fox News Channel reject the crude and irresponsible attempts to suggest otherwise," he added.
"Donald Trump rarely apologises, although in this case, he should."
The exchange was noteworthy because it comes after Trump and Ailes sought to make peace after the contentious debate, which aired on Fox and was watched by a record number of viewers.
Trump launched a Twitter tirade against Kelly the night after the debate, saying she "bombed" as a moderator, was the "loser" of the debate and treated him unfairly for asking crude questions about his previous comments about women.
Trump has soared in the polls since he announced his presidential campaign in June. He comes across to many voters as a straight talker with no filter, antagonising the Washington establishment.
Republican rival Senator Lindsey Graham lashed out at Trump on Tuesday, telling CNN that the real estate mogul's views on women and immigrants did not reflect today's GOP.
"If Donald Trump is the nominee, that's the end of the Republican Party," Graham said.