The Welsh Conservative leader broke Senedd rules by incorrectly describing 20mph as a blanket policy, a committee concluded.
Andrew RT Davies was found to have brought the Senedd into disrepute after calling the default speed limit a "blanket" policy on social media.
The tweet, posted just days after a ruling that the phrase was "imprecise and inaccurate", read: "Another bus route cut thanks to Labour and Plaid's blanket 20mph speed limits."
An investigation by Douglas Bain, the Senedd's standards commissioner, concluded Mr Davies breached the code of conduct by continuing to use the term.
But the South Wales Central MS was cleared of breaking rules around honesty, with Mr Bain saying: "I am satisfied that it was not his intention to deceive anyone."
'False'
He explained that untruthfulness requires an element of deceit, fraud or moral turpitude.
Mr Davies, who has led the Welsh Tories in two stints since 2011, will be formally "censured", which amounts to a slap on the wrists, in the Senedd next Wednesday.
The politician admitted he was aware of the standards committee's conclusion that depicting the default speed limit as a blanket policy was imprecise and inaccurate.
Mr Davies argued he was entitled to use the phrase under Article 10 (freedom of expression) of the European Convention on Human Rights.
But Mr Bain ruled: "When he posted the tweet the member knew – or ought to have known that – although it was not untruthful or dishonest, it was false."
"It was not protected by the enhanced protection afforded to politicians."
'Failed'
Mr Davies, who undertook not to use the term "blanket" until the complaint had been dealt with, asserted the descriptor should, at worst, be tolerated as an "inaccurate exaggeration".
In February, Shaun Haggerty complained to the commissioner, criticising Mr Davies for "continuing to falsely use the term", prompting the investigation.
And in a report that followed on Wednesday, the commissioner found Mr Davies breached rules one (leadership principle) and two (bringing the Senedd into disrepute) of the code.
Mr Bain, who was appointed in 2021 after serving as acting commissioner, said it was incumbent on the leader of the opposition to set a good example.
He wrote: "I am satisfied that when he posted the tweet the member knew or ought to have known that it was 'imprecise and inaccurate' and so false."
"By ignoring the committee's admonition and the guidance given...he failed to give the leadership required of him."
'Birthing people'
Mr Davies was also investigated following a separate complaint, submitted by Anthony Jones in April, similarly centred on social media posts.
The tweet stated: "Vaughan Gething's Labour government is embracing the same extreme ideology as its predecessor. Nothing has changed."
Below was a picture, from the Guido Fawkes political blog, of the ex-first minister and a pregnant woman, captioned: "Welsh Government press release celebrates 'birthing people'."
The complainant described the post as a "blatant lie", warning that it was misleading and dangerous, with the Welsh Government confirming no such press release was issued.
Mr Davies pointed out that a ministerial statement on April 26th referred to 'birthing people' as he again argued he was exercising his right to freedom of expression.
Interviewed under oath, Mr Davies told the commissioner Mr Jones had "serially complained" about his conduct, with four other complaints since 2023.
'Irrelevant'
But Mr Bain did not consider any of the four complaints vexatious and was not satisfied with an explanation that the text had been copied from Guido Fawkes.
"I am clear that is irrelevant," he wrote in his report.
"Members are fully responsible for any quotation they choose to include in a tweet...that has been made clear to members repeatedly."
Mr Bain pointed out that guidance on the code of conduct states Senedd members are expected to reasonably fact check and verify their assertions.
He said: "As a former member of the standards of conduct committee, I consider it inconceivable that he was not aware of that."
The commissioner asked whether Mr Davies accepted that a politician knowingly making a false or misleading statement would be likely to bring the Senedd into disrepute.
'Misleading'
He replied: "If someone deliberately did that, of course, that would be a case of bringing the Senedd into disrepute. But I don't accept that in this case one iota."
Mr Davies described press releases and ministerial statements as synonymous but Mr Bain did not accept this, saying there is a clear distinction.
He wrote: "Whilst I have no doubt the member's tweet...was incorrect and potentially misleading, I am not satisfied on the evidence it can be found to have been untruthful."
However, Mr Davies was again found to have brought the Senedd into disrepute.
The commissioner said: "I am satisfied that the member made no attempt to check the accuracy of the text from Guido Fawkes that he copied into his tweet."
Mr Davies was offered an opportunity to comment.
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