ITV News presenter Alastair Stewart is stepping down from his role following what ITN called "errors of judgment in Alastair's use of social media".
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.
Very sad about the departure of #AlastairStewart. He was a good friend and mentor to me when I started at Carlton TV, and we worked together for more than 27 years. I will miss him.— Mary Nightingale (@nightingaleitv) January 29, 2020
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."
A career on camera
Stewart, from Hampshire, joined ITN as industrial correspondent in 1980.
He also served as a foreign correspondent, and was sent to Washington in 1990 and then to Saudi Arabia, from where he anchored ITV's news coverage of the first Gulf War for two months.
Behind the news desk, he hosted ITV's lunchtime and early evening news, as well as News At Ten and Channel 4 News. His other shows included the long-running Police, Camera, Action!, which was made using police footage of dangerous driving.
Stewart was named presenter of the year by the Royal Television Society in 2005, and was awarded an OBE the following year.
He often anchored ITV's coverage of elections and royal weddings, and in 2010 he moderated the UK's first-ever televised general election leaders' debate, between David Cameron, Gordon Brown and Nick Clegg.