EXCLUSIVE: Princess Kate Middleton's Terror — Radar Reveals How 'Horrifyingly Violent and X-Rated' Threats are 'Being Hidden From Future Queen'

Security teams working across the Royal Household, including specialists linked to the U.K.'s Fixated Threat Assessment Centre, closely monitor the situation as part of ongoing efforts to manage risks posed to senior royals.

The Princess, who remains one of the most visible members of the royal family following her return to public duties after cancer treatment, has long attracted intense attention – but insiders said the tone of some communications has grown more alarming in recent years.

A source familiar with royal operations described the nature of the letters being intercepted, saying: "The scale of correspondence Catherine receives is enormous, and while much of it is supportive, there is a deeply troubling subset that is inappropriate, obsessive, or outright disturbing.

"Staff working in the mail rooms are encountering letters that cross boundaries in ways that can only be described as stalker-like."

Insiders added strict protocols are now in place to prevent the Princess from being exposed to the most extreme material.

One source said: "There is effectively a secret, and very strict, policy to filter out and withhold the worst of it from Catherine entirely. The language being used internally is about protecting her sanity – making sure she is not confronted with the most horrifyingly violent and X-rated messages that come through."

Another insider said the volume and nature of the correspondence require constant vigilance.

They said: "Some of these letters are graphic, aggressive, and deeply unsettling, and they are arriving with concerning frequency. Teams are trained to identify anything that crosses into threatening territory and ensure it is dealt with immediately, without ever reaching her.

"Kate does not need to know their contents as experts are handling the monitoring of any individuals behind it. It is better she doesn't know the details.

"There are dedicated teams whose role is to assess risk and intercept anything problematic. Catherine is shielded from the worst of it because exposing her to that kind of content repeatedly would be incredibly damaging to her mental health."

The issue of fixated individuals targeting public figures has long been a concern for security services. Historically, Buckingham Palace has received thousands of letters annually, with a proportion linked to individuals exhibiting obsessive or unstable behavior.

Experts suggest around 40 percent of the most serious stalking cases in Britain involve public figures, including members of the royal family. Dai Davies, former head of royal protection for the Metropolitan Police, has previously warned of the risks posed by such individuals.

He said: "They give rise to a greater likelihood of an incident. If you look back through history, the greatest threat to the Royal family apart, from the I.R.A. has been fixated individuals.

"Prevention is one of the key areas because in about 70 percent of cases where there have been attacks, the fixated person has indicated before the incident that they are going to do something."

Those close to Catherine said the reality of such attention remains an ongoing challenge.

A friend said: "The truth is that Kate's position brings with it an enormous range of attention. While there is a great deal of warmth and admiration, there is also a darker side that can be intrusive and unsettling.

"It is something she is aware of, but that does not make it any easier when that attention crosses a line into something more concerning."

Palace aides said security teams continue to monitor individuals considered high risk, working alongside health services and law enforcement to reduce the likelihood of escalation, while ensuring the Princess is protected from the most extreme threats.