Andrew Mountbatten-Windsor has been released under investigation following his arrest on suspicion of misconduct in public office.
On Thursday morning, Thames Valley Police said it had arrested a man in his 60s in Norfolk and was carrying out searches at addresses in Berkshire and Norfolk.
King Charles III said the "law must take its course" in response to Andrew's arrest and that the police had his "full and wholehearted support and co-operation".
Police had previously said they were considering investigating Andrew over allegations relating to his association with the late sex offender Jeffrey Epstein and were reviewing allegations he shared confidential material.
Communications between the pair featured among the tranche of files released by the US government.
