EXCLUSIVE: Princess Kate 'Mortified' Over Prince William's Latest 'Porn Star-Style' Fashion Plan

Dressed in a full camouflage uniform and a regimental beret in his role as Colonel-in-Chief, William spent time speaking with officers and their families, observing training exercises, and discussing military life.

It was during a light-hearted exchange with officers in the mess the subject of facial hair arose, prompting the prince to joke about changing his own appearance.

Lieutenant Jack Austin from South Manchester, who was sporting a mustache, said: "It started in Estonia – a few of us officers grew mustaches and it snowballed from there. I brought it up when I was chatting to the prince, and he said, 'Maybe I should get rid of the beard and just have the mustache.'"

The comment drew laughter at the time, but insiders said it has since taken on a life of its own behind palace doors.

One source close to William and Catherine told us, "Kate was absolutely mortified when she heard about the idea of a porn star-style mustache on William. She has always disliked that look and thinks it would completely change William's image in a way she finds deeply unappealing."

The insider added, "She has joked that he would look like a cheesy 1970s porn star if he went through with it, and it is not something she would support. For her, it crosses the line from playful into something that feels out of step with how she sees him. She is just praying he does not follow through."

The exchange about facial hair took place as William carried out a series of engagements with military personnel, including conversations with female personnel about their experiences in the Army and its zero-tolerance policy on unacceptable sexual behavior (USB).

Captain Maria Bell said: "The Army's moved forward a lot over the past couple of years as regards the USB policy that's come out and signposting."

She added: "The USB course is great, it's there, but more education, more training around that, to chains of command, to within units, not just a tick-box exercise… I think more investment needs to go into the training, education around the subject, what's inappropriate, what's not, just so it's very transparent, no grey areas."

William also joked with Maxine Debonnaire and her son, Lieutenant Harris Debonnaire, about his love of sport, confirming his preference for football over rugby and discussing his support for Aston Villa.

Harris, who also wore a mustache, told the prince his facial hair began during the Movember campaign, and it "just grew on me."

The visit formed part of William's ongoing role supporting the armed forces, including earlier trips to Estonia, where he met the same unit during deployment.

While the mustache remark was made in jest, sources said it has nevertheless sparked a firm response at home, with Catherine said to be hoping the idea remains nothing more than a passing joke.