The 70-year-old appeared on the show to discuss her new book, Meghan and Harry: The Real Story, which will be released later this week.
However, as soon as Lady C joined the show via video call, tensions started to rise in the ITV studios.
Phillip wondered how Lady C gained access to royal informants after her controversial biography about Diana, which was released in the early 90s.
Lady C snapped: “That shows how ignorant you are if I may point out.”
She added: “My Diana biography was the most believable of all the biographies.”
Getting his own back, Phil called Lady C a “gossip,” as he quizzed her about the story behind her new book about Prince Harry and Meghan Markle.
Claiming to be a private person, Lady C said: “I do not leak like a sieve, I am not a Meghan or a Harry.”
Phil retorted: “But you are a gossip?”
Lady C replied: “Not when it suits me. If you had any respect for the truth you would be able to appreciate that.”
Closing the interview, Phil took one last swipe, saying: “We’ve got to leave it there, it’s been fascinating if a little patronising.”
On Twitter, viewers were left divided over the confrontation, as they witnessed the awkward clash unfold on their screens.
One asked: “How does Lady C get away with calling people ignorant?”
Though a second countered: “I watched this interview & I don’t think it was Lady C who was patronising…”
And a third joked: “I’d love to have heard what Phil said about Lady C during the ads…”
This Morning airs on weekdays at 10am on ITV.