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.
ITV presenter Julie Etchingham wrote on Twitter: "So sad to learn this - we have worked on many big stories together & Al is a trusted friend and guide to many of us."
Fellow newsreader Mary Nightingale added that she was "very sad" about his departure.
Michael Jermey, ITV director of news and current affairs, said in a statement: "Alastair has been a long-standing, familiar figure to viewers of ITV News, both reporting and presenting with distinction. We wish him the very best for the future."
ITN's chief executive Anna Mallett added: "We would like to recognise Alastair's contribution as one of the UK's foremost journalists and TV presenters and to thank him for his commitment to delivering high-quality broadcast news over many years."