Fox News has apologized for a guest who called environmental activist Greta Thunberg mentally ill, and said he would never appear on the network again.

Michael Knowles of "The Daily Wire" made the comment Monday during a segment on a Fox evening newscast, also saying Thunberg was being exploited by her parents and the left wing.

Fox had no comment Tuesday about its own prime-time host, Laura Ingraham, who likened Thunberg to a murderous child cult leader from a Stephen King short story.

The network said in a statement that Knowles' comment was disgraceful.