Prince William feared Prince Harry was being "brainwashed" by his therapist.
The Duke of Sussex claimed his older brother got in touch with him after the 2019 Well Child awards after seeing him grow emotional on television and started questioning his state of mind.
Harry wrote in his new memoir 'Spare' that the exchange sparked a 72-hour argument and he even urged William to join him at one of his therapy sessions to prove there was nothing sinister going on.
He wrote: "He said I wasn't well. He said again that I needed help.
"I reminded him that I was doing therapy.
"In fact, he'd recently told me he wanted to accompany me to a session because he suspected I was being 'brainwashed'."
Harry, 38, claimed he invited his 40-year-old brother along to a joint meeting with his therapist, telling him it would be "good for you [and] good for us" but the Prince of Wales refused.
The BetterUp CIO - who has children Archie, three, and Lili, 19 months, with wife Meghan, Duchess of Sussex - went on to claim William believed his desire to step down from royal duties was because he "was unwell" and as a result thought Harry "was unwise" to want to move overseas.
But Harry thinks he had simply become a "stranger" to his brother after spending "months" working on himself.
He wrote: "After months of therapy, after working hard to become more aware, more independent, I was a stranger to my older brother.
"He could no longer relate to me - tolerate me. Or maybe it was just the stress of the last few years, the last few decades,
finally pouring out."
And the prince admitted he sometimes reads back his exchanges with William and is amazed at how much their relationship has changed.
He recalled thinking about the text messages: "How did we get there?"