He said: “From day one he began to interfere with the editorial content of the show. Fern always left it to the editor, a mistake she attempted to rectify, but too late.
“His stranglehold was too tight by the time she recognised what he was up to. Their ability to work together was up there with the Eamonn Holmes/Anthea Turner friction.”
Jon, who also spoke out against former client Esther McVey earlier this month amid her public spat with Lorraine Kelly, added: “[Phil] befriended Simon Shaps, the then boss of the network centre. After this friendship, all bets were off ... he just got what he wanted.”
A spokesperson for Phil did not immediately reply to HuffPost’s request for comment on Jon Roseman’s interview, while a spokesperson for This Morning had nothing to add.
Fern presented This Morning alongside Phillip between 2002 and 2009.
But tensions between them bubbled over live on air last year when Fern made an appearance via video link during a special episode to mark This Morning’s 30th anniversary.
A public exchange on social media then followed, amid claims Fern had been snubbed from a special Bafta event that was part of the celebrations – although it was later revealed her invitation had simply not been passed on by her team.
Jon Roseman’s comments about Phillip follow claims he blocked Amanda Holden from standing in for Holly on the ITV daytime show last year.
The Sun’s story claimed Phillip had influenced the decision to hire Rochelle Humes present alongside him while Holly co-hosted I’m A Celebrity, with Amanda reportedly complaining about Phil to top ITV boss Kevin Lygo at a meeting last September.
It was also alleged that a second ITV worker had complained about Phil being “overbearing” and contributing to a “toxic” work environment.
However, ITV was quick to deny Phillip had been instrumental in Rochelle’s hiring, saying in a statement: “Presenter line-ups on This Morning change regularly. Final decisions are made by producers, not presenters.
“Phillip is a much-loved broadcaster and part of the ITV family. He’s a consummate professional and held in high regard at ITV.
“Amanda is also held in high regard as a judge on one of our biggest shows,” they added.
Writing on Twitter, Phillip called the rumours “hurtful and wildly untrue”, while Amanda also commented on the reports on her Heart Breakfast Show, claiming she had offered Phil an olive branch by inviting him out for coffee but had received to reply to her message.
Headlines about their soured relationship also came a week after reports of a falling out between Phil and Holly over a Sunday Times Magazine cover.