In a statement, the veteran broadcaster, 67, said: "It was a misjudgement which I regret."
ITN said he had breached editorial guidelines, but did not elaborate on the reasons for his departure.
He added: "It's been a privilege to bring the news to households throughout the UK for the past 40 years."
Stewart joined ITV in 1980 and began fronting News at Ten nine years later.
The newsreader's Twitter account has now been deactivated.