ITV News host Alastair Stewart is stepping down from presenting duties "following errors of judgment" in his use of social media.
The veteran presenter, 67, said in a statement that he made a "misjudgement which I regret".
ITN said that Stewart's use of social media "breached" its editorial guidelines.
The Daily Star reports that Stewart said on Twitter that "there are some absolute p****s at the BBC".
However, other people believe the offending remark came several months ago when the TV presenter called someone an "absolute p****" during a Twitter spat.
Stewart's Twitter account has now been closed down.
The decision to quit "is supported by both ITV and ITN".
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."
Stewart has been a staff reporter and presenter on ITV News programmes.
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."
People on social media were outraged that Stewart had quit presenting over such a seemingly minor infringment.
Glen @GJPatterson said: Please don't tell me that Alastair Stewart has been 'stood down' from ITV news merely for calling someone a 'p****' on social media?? Is that it?!"
Kirsty Bennetta @Kirstyb1989 said: "Alastair Stewart feeling that he had to step down for allegedly calling someone a p**** online is the most pointless and stupid thing I’ve ever heard! A man with a fantastic career in broadcasting (the longest standing might I add) feeling the need to apologise for his misjudgement and have to end his 40 year career on such a low because SOMEONE was offended. Seriously people... There’s a difference between malicious hate towards someone and trying to deliberately offend and just expressing an emotive opinion through differences.”
However, a new theory later emerged over Stewart's mystery departure.
Many believe the TV star decided to call it a day after he used the word "ape" when quoting Shakespeare in a tweet sent to a black Twitter user. The incident took place just weeks before he announced his shock resignation.
Stewart and Twitter user Martin Shapland had argued on Twitter over how the Crown was funded by the taxpayer.
The newsreader then quoted from Shakespeare's Measure for Measure.
He wrote: "But man, proud man, Dress'd in a little brief authority, Most ignorant of what he's most assur'd - His glassy essence - like an angry ape, Plays such fantastic tricks before high heaven.
"As makes the angels weep; who, with our spleens, Would all themselves laugh mortal."
Mr Shapland later retweeted the comment and branded Stewart a "disgrace".
He said: "Just an ITV newsreader referring to me as an ape with the cover of Shakespeare. Measure for measure, Alistair is a disgrace."
Neither ITN nor Stewart have revealed what the "misjudgement" was that caused him to step down.
However, Mr Shapland appears to offer a clue that he was involved in the newsreader's decision with his Twitter page profile now saying: "Tweet in haste, repent at leisure."
