Eton College's former head of English has warned that the school's leadership is "teetering on the edge of farce".
Joseph Francis, who taught at Eton for 20 years, said that the authority of both the head master and provost had been "undermined" by the free speech row that has engulfed the £42,500-a-year school for the past week.
Mr Francis is the first former senior master to speak out publicly about the school's handling of the affair, and his intervention comes after a current master accused Eton of "indoctrinating" students.
Last week, a row broke out after The Telegraph revealed that a master had been dismissed for gross misconduct after recording a lecture which questioned "current radical feminist orthodoxy".
The controversial lecture was part of the Perspectives course taken by sixth form students to encourage them to think critically about subjects of public debate.
Will Knowland alleged that he was banned from delivering the lecture to pupils and then dismissed after he refused to remove a video of it from his personal YouTube channel.
Eton has said the dismissal was "not a matter of free speech" but one of "internal discipline". An internal panel, chaired by the school's vice-provost, will meet next week to consider Mr Knowland's appeal.
But Mr Francis, who left Eton last year to set up a network of low-cost independent schools called Scholar 6, has warned that the outcome of the appeal panel will "undermine Eton's head and provost whichever way it goes".
Writing in The Telegraph, he said: "Knowland, with heavyweight support from two Harvard professors, has won key intellectual arguments.
"If his refusal to take down a video lecture about masculinity is deemed gross misconduct, he becomes a martyr to hundreds of boys, thousands of Old Etonians and millions of random bloggers. If the dismissal is overturned, Eton's leadership teeters on the edge of farce."
Professor Steven Pinker, an expert in experimental cognitive psychology at Harvard University, has written to Eton's provost in support of Mr Knowland.
"In the name of honest and rigorous discussion of ideas, and respect for the ability of Eton students to evaluate and debate them, I urge you not to fire Mr Knowland," he said.
Richard Wrangham Moore, a research professor of biological anthropology at Harvard, also wrote to Baron Waldegrave of North Hill saying he was "astonished" to learn of Mr Knowland’s dismissal.
Prof Moore argued that the school's actions "threatened Eton's reputation for openness, intellectual honesty, scholarly courage, and promotion of independent thought".
Mr Francis, who used to be head master at Hampton Court House, said it was a "mistake" to insist that Mr Henderson remove the video.
"By instructing Knowland to withdraw his lecture, he undermined the principle of controversy behind the Perspectives course," he said. "Knowland dug in and secured the higher ground."
Former and current pupils have criticised Eton for dismissing Mr Knowland, with more than 2,000 signing a petition that accused the school of "institutional bullying". But some parents have come to Mr Henderson's defence, saying that he has their total support and accusing Old Etonians of launching a smear campaign against him.
Earlier this week, a current Eton master resigned one of his positions in protest and broke ranks to attack the school's "indoctrination" of students.
Dr Luke Martin, who teaches Divinity at the 580-year-old institution, recently stood down from his role as the master in charge of Perspectives and said he was beginning to "lose faith" in Eton's ability to promote independent thinking among pupils.
An Eton spokesman said: "The school was advised by a specialist barrister that the content in Mr Knowland's video lecture – which appeared online with a reference to his employment at Eton – could very well breach a number of the school's policies as well as the school's legal and regulatory obligations.
"That gave the school no choice but to ask for it to be taken down. The school asked Mr Knowland to remove it, pending further discussion. He refused several requests to do so.
"A second lawyer conducted an independent investigation into his conduct, which led to a disciplinary hearing heard by a panel of senior teachers which did not include the head master. It determined that the teacher's actions represented gross misconduct which should result in dismissal. That decision is now the subject of an appeal.
"Many teachers have written to the provost or the head master supporting the line the school has taken. They have written in the belief that such correspondence is private."