Amid a backlash from Labour supporters, Holly Rigby, a teacher who is regularly interviewed by the media about the party, said people had to read Rabbi Mirvis "in the context" of being a supporter of the Conservative prime minister and “an uncritical supporter of Netanyahu and the violent oppression of Palestinians by Israel".
Her tweet, for which she was widely condemned, cited the fact he had "congratulated" Mr Johnson on becoming Conservative leader - a common formality whenever a new party leader is chosen - and the fact he said in August 2014 that Israel had "understandly and justifiably defended her citizens" by taking military action in Gaza.
She wrote that the chief rabbi "does not represent all of Britain's Jews" and had "weaponised antisemitism to delegitimatise those who want justice for Palestine".
Ms Rigby, who has yet to respond to a request for comment and deleted the three tweets, appeared as recently as last week on BBC's Politics Live to speak for Labour.
The Labour Party has been aproached for comment.
Ms Rigby also provides training on campaigning to members of Momentum, a pro-Corbyn campaign group within Labour.
After she attacked the Chief Rabbi, the Guido Fawkes website unearthed a video from March in which Ms Rigby also defended disgraced former Labour MP Chris Williamson after he said the party had been “too apologetic” over anti-Jewish racism.
Mr Williamson was suspended in February over the comment. He was reinstated in June but was suspended again after a backlash from MPs, peers and Jewish groups. He remained suspended when the election was called and is not a candidate.
However Ms Rigby told the audience as a proud “outspoken Corbynista…antisemitism has been used as a smear for our movement".
“It is little surprise that, when Chris Williamson told a packed meeting of Labour Party members that Labour had been too apologetic when it came to its strategy of dealing with the right's weaponisation of antisemitism that he was met with rounds of enthusiastic applause from Labour Party members in the audience.”
She went on to defend Pete Willsman, the senior Labour figure whose rant about Jewish "Trump fanatics" was exposed by the JC.
Mr Willsman was later suspended from the party after he was recorded claiming the party's antisemitism crisis is being manufactured by Israel.
Ms Rigby, who is a teacher at School 21 in east London, said: “When Momentum took the misplaced decision to drop the NEC candidate Pete Willsman following accusations from the right of the party that he had made antisemitic comments, Labour party members were the ones who ignored Momentums directive and voted for him regardless.”
She also described Labour’s decision to adopt the International Holocaust Remembrance Alliance’s (IHRA) definition of antisemitism in full after months of wrangling as “shameful.”
She said: “I think that I speak for many in the Labour Party when I say that if expressing the political truth that the current Israeli state is a racist endeavour means being expelled from the Labour party then so be it.”
The party had been in a standoff with the Jewish community after it refused to adopt all the examples of Jew-hate in the IHRA definition of antisemitism, including ones that relate to criticism of Israel.
The definition cites claiming that "the existence of a state of Israel is a racist endeavour" as an example of potential antisemitism.
In a statement School 21 said it was taking reports of her comments "seriously. We are looking into the matter using our internal policies and processes and will take any appropriate action that is necessary.”
A Momentum spokesperson said: "Trainers are sourced from a network of more than 30 activists across the country.
“There is no place for antisemitism in Momentum and we are absolutely committed to combating antisemitism wherever and whenever it occurs."
It said all "complaints concerning a Momentum member are investigated fully. If a member is found to be in breach of our code of ethics or constitution, appropriate disciplinary action is taken."
