In a statement shared with the PA news agency, Mr Thomas blamed the cancellation on "vicious outside pressure".
He said: “An outside pressure group has declared war on our film. They wrote to the festival's sponsors... and whipped up huge storm of complaints about the film claiming, without any foundation whatsoever, that the film is antisemitic."
Jeremy Corbyn speaks on stage on day 3 of the Glastonbury Festival 2017 (Photo: Getty Images)
He continued: "The claim that the film is antisemitic is a total smear. The festival organisers even had a lawyer examine the film who pronounced it totally devoid of antisemitism."
The documentary, according to Platform Films, claims Mr Corbyn was a victim of a "concerted smear campaign" and that current Labour leader Sir Keir Starmer was "waging a witch-hunt in the party".
Mr Thomas continued: “Journalists, who know very well what's going on, have to stand up and call this out for what it is: rank censorship.
"The problem is, if they do, they fear being accused of antisemitism.
"But if enough of us stand together the accusation won't stick and this madness will end."
A statement attributed to Glastonbury said: "Although we believe that the Pilton Palais booked this film in good faith, in the hope of provoking political debate, it's become clear that it is not appropriate for us to screen it at the festival.
"Glastonbury is about unity and not division, and we stand against all forms of discrimination."
The Board later said it was pleased the film would no longer be shown at the festival. Writing on Twitter, they said: “We are pleased that in the wake of a letter we sent earlier today, @glastonbury have announced the cancellation of the screening of this film. Hateful conspiracy theories should have no place in our society."
The film was expected to be shown on Sunday, June 25 at Glastonbury's Pilton Palais cinema.
The film, which is linked to on Glastonbury’s website is described as exploring: “a dark and murky story of political deceit and outrageous antisemitic smears.”
The film’s contributors include the filmmaker Ken Loach, former MP Chris Wiliamson and sacked Bristol lecturer David Miller.
Also involved in the film is Andrew Murray, a close adviser to Mr Corbyn who in 2005, authored an article in which he claimed that the roots of the 9/11 terror attacks lay in “Zionist colonialism” of the Balfour Declaration.
