The veteran presenter, 67, said in a statement he had made a “misjudgement which I regret”.
ITN said that 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 70s.
Recently he has worked as a part-time freelance presenter, mainly on the lunchtime news and weekend programmes.
Michael Jermey, 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.”
Additional reporting by PA.