London, UK - Britain's King Charles III said Thursday, "the law must take its course" after police arrested his brother, the former prince Andrew, on suspicion of misconduct when he was a trade envoy.

"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," Charles said in a rare personally signed statement.

"In this, as I have said before, they have our full and wholehearted support and co-operation," he noted, before adding, "Let me state clearly: the law must take its course."

The arrest of a royal family member is unprecedented in Britain's modern era.

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It was a humiliating new blow for the ousted prince, who was stripped of his titles last year and was marking his 66th birthday Thursday in custody.

Police said they were also searching two properties, with the BBC reporting one was Andrew's former home, Royal Lodge, on the monarchy's Windsor estate west of London.

The other was his residence since the start of this month on the king's private Sandringham estate in Norfolk, eastern England, where his arrest occurred, according to British media.

Widely published images showed a fleet of unmarked cars, believed to be police vehicles, arriving there early Thursday.

"We have today (19/2) arrested a man in his sixties from Norfolk on suspicion of misconduct in public office," Thames Valley police said, without naming the suspect, as is common practice in the UK.

"The man remains in police custody at this time," the force added.

Andrew's ties to convicted US sex offender Epstein have caused a spectacular years-long fall from grace.

His arrest follows new revelations last week that the ex-prince appeared to have sent potentially confidential documents during his time as a UK trade envoy.