Following the arrest — the first of a senior royal in modern history — the King addressed the situation publicly.
As reported by PA Media, Charles expressed his “deepest concern” and emphasised that “the law must take its course”.
The monarch said: “I have learned with the deepest concern the news about Andrew Mountbatten-Windsor and suspicion of misconduct in public office.
“What now follows is the full, fair and proper process by which this issue is investigated in the appropriate manner and by the appropriate authorities.
“In this, as I have said before, they have our full and wholehearted support and co-operation.”
He continued: “Let me state clearly: the law must take its course. 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. Charles R.”
Charles had previously commented on the allegations involving his brother. A Buckingham Palace spokesperson said at the time: “The King has made clear, in words and through unprecedented actions, his profound concern at allegations which continue to come to light in respect of Mr Mountbatten-Windsor’s conduct.”