Prince William and Kate Middleton are said to be "surprised and upset" by their chief bodyguard's decision to leave his royal duties and become head of security at Berkshire's Broadmoor psychiatric hospital.
Chief Inspector John Hourihan, who has worked with the couple for five years, accompanied them on numerous high-profile trips including their visit to Borneo as part of their South East Asia tour last year.
John is thought to have been tempted by a lucrative offer with a private security firm at Broadmoor, the famous high-security psychiatric hospital.
He previously worked as the Personal Protection Officer for the Earl and Countess of Wessex before being drafted in in 2008 for William and Kate. Until then, Kate had not had round-the-clock protection.
John's appointment sparked rumours of an impending engagement between William and Kate because the move echoed when Princess Diana was appointed chief inspector, Graham Smith, as head of her protection team shortly before her engagement to Prince Charles was announced.
John, who managed two teams of four bodyguards, is known for his charm and discretion. The bodyguards are experts in marksmanship, close protection duties, first aid and the essential diplomatic skills.
It is not known who will replace John.