The brother of Princess Diana, Charles Spencer, said her son, Harry, and his wife, Meghan, should be given official security.
Earl Spencer said the media now treats the couple the same way they treated Diana in the years leading up to her death.
His upcoming book about Diana is critical of the now-King Charles.
Diana died aged 36 in a 1997 Paris car crash as photographers pursued her on motorbikes, sparking an outpouring of grief and a reckoning over whether the media bore some responsibility after it splashed details of her life across front pages for years.
In the book, Earl Spencer accuses Diana's former husband, the then-prince of Wales, of sounding "giddily elated" in the hours after her death and said a Charles had told Spencer that the world would forget Diana soon enough.
Asked by the BBC how he could prove the conversations had taken place, Earl Spencer said he had included a specific line in his much-lauded eulogy at Diana's funeral to counter the comments, ending it by saying she would "never be extinguished from our minds".
Diana's family accused the royals of showing a lack of support or care towards the late princess before her death, a charge that was repeated by Harry and Meghan about their personal experience before they quit royal duties in 2020 to move abroad.
Earl Spencer said his revelations about Charles would not come as a surprise to Harry or his older brother, heir-to-the-throne Prince William, and said while he knew many more details about the king he had chosen not to include them.