A royal source has revealed the real reason why Prince Charles and Prince Harry are still not speaking.

"He gets told, 'He's not available right now,'" a friend close to the Duke of Sussex told People in an exclusive interview for the magazine's latest cover story. 'He doesn't answer his phone. He is trying to get in touch with the king about his health, but he is not answering those calls either."

According to a source close to Prince Harry, who said he is familiar with the issue, Prince Charles' relationship with his youngest son "deteriorated" after the monarch's cancer diagnosis.

"People close to Prince Harry say the monarch no longer answers his son's phone calls or responds to his letters," People reports, adding that the king has not responded to Prince Harry's repeated requests for help regarding the safety of his children, Prince Archie and Princess Lilibet.

"Harry is frightened and feels that the only person who can do anything is his father," a royal source told the magazine. Another source added that the Duke of Sussex is "determined to protect his family at all costs."

In a recent interview for the ITV documentary "Tabloids on Trial," the Duke of Sussex revealed why he says he is still unwilling to bring his family, especially his wife, Meghan Markle, back to the UK.

"All it takes is one actor, one person who reads this stuff, to act on what they read," Prince Harry said at the time, adding that it is "still dangerous" to bring his family across the pond.

"Whether it's a knife, acid, whatever it is, these are things that really concern me," Prince Harry continued, referring to ongoing security concerns and threats to his family.

"It's one of the reasons I'm not bringing my wife back to this country," he explained.

Indeed, Prince Harry visited the UK in person to mark the 10th anniversary of the Invictus Games, a Paralympic-style competition honoring veterans, but mostly skipped it due to ongoing security concerns.

The prince eventually decided to attend in person, but his wife and two children were absent.

During Prince Harry's most recent visit to the UK, he reportedly attempted to meet his father in person.

"In response to many inquiries and continuing speculation as to whether the Duke will meet his father during his stay in the UK this week, we regret to say that due to His Majesty the King's packed program, he is unable to meet him," a spokesman for Prince Harry said, The Telegraph reported at the time.

"The Duke, of course, is aware of his father's timetable and various other priorities and hopes to meet soon," the spokesman added.