King Charles on Thursday issued a statement after his younger brother Andrew Mountbatten-Windsor was arrested on suspicion of misconduct in public office, calling for the "full, fair and proper process by which this issue is investigated."
The monarch, who stripped Andrew of royal titles and evicted him from Royal Lodge in Windsor, assured the people, "In this, as I have said before, they have our full and wholehearted support and co-operation."
"Let me state, clearly: the law must take its course," said King Charles III in a statement issued by Buckingham Palace.
King Charles said, "I have learned with the deepest concern the news about Andrew Mountbatten-Windsor."
He added, "As this process continues, it would not be right for me to comment further on this matter, Meanwhile, my family and I will continue in our duty and service to you all."
Former prince Andrew was arrested Thursday on suspicion of misconduct during his time as a trade envoy, as UK police investigations into allegations emerging from the Jeffrey Epstein files gathered pace.
The arrest of a royal family member is unprecedented in Britain's modern era, as police said they were also searching two properties -- in eastern England and west of London -- as part of their probe.
It was a humiliating new blow for the ousted prince, who was last year stripped of his titles, and Thursday marked his 66th birthday as he was taken into custody.
"We have today (19/2) arrested a man in his sixties from Norfolk on suspicion of misconduct in public office," Thames Valley police said in a statement, without naming the suspect, as is common practice in the UK.
Officers "are carrying out searches at addresses in Berkshire and Norfolk," the force added, noting "the man remains in police custody at this time".