Prince Harry needed to seek permission from his grandmother, the late Queen Elizabeth II, to keep his beard when he married Meghan Markle.
The Duke of Sussex told ITV's Tom Bradby, before the release of his memoir Spare on Tuesday, that it had been the source of an argument with his brother, Prince William.
He said the beard was a "shield" to his anxiety, he feared his bride might not recognise him without it, and that his brother felt it was unfair that he had been previously made to shave off a beard.
It was one of the more obscure subjects Prince Harry raised in the interview, which also covered his years of disappointment at his family's lack of support and his anger at the British press.
Bradby said the argument over the beard was "one of the most puzzling in the book".
In a voiceover, he said: "Harry claims he went to see his granny, the Queen, to ask, as required, for permission to keep his beard for his own wedding. She was amenable, William was not.
"There was then an argument, Harry says, between the two brothers that went on for a week and ended, he claims, with his brother ordering him, as the heir to the spare, to shave it off."
Bradby then said to the duke: "I mean I just read that and thought ‘What — what is that really about?’ because that is not about 'to beard or not to beard'."
The duke replied: "I mean I refer to it as heir-spare but also older brother-younger brother, right?
"There’s a level of competition there. And again, writing this, I remembered that William had a beard himself and that granny and other people told him that he had to shave it off.
"The difference for me, if there was a difference, as I explained to my grandmother, that this beard that I’m still wearing, felt to me at the time like the new Harry. Right?
"As almost like a shield to my anxiety. That was the time of my life when I grew my beard.
"I think William found it hard that other people told him to shave it off, and yet here I was on my wedding day wearing military uniform, no longer in the military, but thinking as though I — believing as though I should shave it off before my wedding day.
"And I said ,‘Well, I don’t believe that Meghan’s going to recognise me if she comes up the aisle and sees me beardless’.
"I would feel very, very different without my beard, and that’s hard for people to understand who’ve never grown a beard, but hopefully those beard people out there will go ‘Yeah, no, I fully get that, I can understand’."
Prince Harry's account prompted comedian David Baddiel to question whether the topic warranted a “constitutional crisis”.
“As a man with a beard I relate to Harry not wanting to shave off his beard,” Baddiel tweeted.
“But not to the idea that having or not having a beard can become some kind of constitutional crisis.”
The 90-minute interview was packed with the duke's criticism of his family.
Prince Harry's autobiography: What to expect
Referring to the British press, he accused members of his family of “getting into bed with the devil” to rehabilitate their image, and his father King Charles of sacrificing his son’s interests in favour of his own.
The duke also criticised family members for a “really horrible reaction” on the day Queen Elizabeth died, with leakings and briefings.
He also said he loved his father and brother, the Prince of Wales, but: “At the moment, I don’t recognise them, as much as they probably don’t recognise me.”
“Nothing of what I’ve done in this book or otherwise has ever been any intention to harm them or hurt them."
His book has sparked a furore over his claims that Prince William physically attacked him, and his admission that he killed 25 Taliban members during the Afghanistan war.
Prince Harry and his wife, Meghan, through the years - in pictures
