The veteran presenter, 67, said in a statement he had made a “misjudgement which I regret”.
ITN said that Mr Stewart’s use of social media “breached” its editorial guidelines – but gave no confirmation on what had led to the decision.
The decision to quit “is supported by both ITV and ITN”, the broadcaster added in a statement
Mr Stewart said: “It was a misjudgement which I regret, but it’s been a privilege to bring the news to households throughout the UK for the past 40 years.”
The veteran newsreader has been a staff reporter and presenter on ITV News programmes since the Seventies.
Recently he has worked as a part-time freelance presenter, mainly on the lunchtime news and weekend programmes.
Michael Jermey, the director of news and current affairs at ITV, said: “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 chief executive Anna Mallett said: “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.”
While it remains unclear what infraction had led to the parting of ways between ITV and the veteran newsreader, Mr Stewart was known to use his Twitter account for combative debates with both journalists and members of the public.
In one exchange three weeks ago Mr Stewart faced criticism for responding to a black Twitter user with a Shakespeare quote which appeared to refer to his counterpart as “like an angry ape” – responding with an emoji implying innocence when it was raised that some may consider his use of the phrase racist.
The lines from Measure for Measure – which are often highlighted for their reference to a position of expertise being taken on by “little brief authority” – were used by Mr Stewart on a number of occasions on the social media site.
An ITN spokesperson approached by the Independent declined to comment on the precise reason for Mr Stewart’s departure.
Additional reporting by PA
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