8 hours ago Duncan Walkerand Tom McArthur Andrew Mountbatten-Windsor has been arrested on suspicion of misconduct in public office.
It comes after Thames Valley Police said it was assessing a complaint over the alleged sharing of confidential material by the former prince with late sex offender Jeffrey Epstein.
The former prince has consistently and strenuously denied any wrongdoing in relation to Epstein.
He has since been released under investigation.
Andrew was seen leaving Aylsham police station on Thursday evening.
Why has Andrew Mountbatten-Windsor been arrested?
Mountbatten-Windsor was arrested at about 08:00 GMT on Thursday on the King's Sandringham Estate in Norfolk, where he has been staying.
Officers arrested the King's brother on his 66th birthday.
It is understood that neither the King nor Buckingham Palace was informed in advance of Mountbatten-Windsor's arrest, by officers from Thames Valley Police.
We don't know the specifics of the allegations of misconduct in public office being investigated. However, previously reported allegations include that he had shared reports from trade visits, forwarded a confidential briefing on investment in Afghanistan, and passed a Treasury briefing to a personal business contact.
BBC News UK and home affairs correspondent Daniel Sandford said: "There has to be an assumption that part of this is going to be the sharing of documents within the Epstein files - between Andrew and Jeffrey Epstein himself.
That's when Thames Valley Police got involved. "
But Sandford said it was possible that police were not only focusing on emails seen widely in the Epstein files.
"Firstly, there's three million documents in the files, so there may be some the media have missed.
But secondly, Buckingham Palace said they would support Thames Valley Police in their inquiries. "And that potentially means the police have gone to the Palace, or could go to the Palace, asking to see evidence of communications between Andrew and others. "
Assistant Chief Constable Oliver Wright of Thames Valley Police said: "We understand the significant public interest in this case, and we will provide updates at the appropriate time. "
The force said it would not be naming the arrested man "as per national guidance".
In a later update on Thursday evening, Thames Valley Police confirmed the arrested man "has now been released under investigation".
It is the first time the former prince, who has faced numerous allegations over his links to Epstein, has been arrested.
What is misconduct in public office?
Misconduct in public office is a really complicated offence, writes BBC News home and legal correspondent Dominic Casciani.
It essentially boils down to an allegation that someone who was doing a job on behalf of the British public did something seriously wrong, knowing it to be wrong.
There are four "elements" or factors that police must focus on during their investigation so that prosecutors can later decide whether or not someone should be charged.
First, the police must establish whether the person they're investigating was a "public officer" and the incident in question was plausibly part of those duties.
If that's agreed, detectives will then look for evidence that the incident in question saw the suspect "wilfully" neglecting to perform their duty or wilfully misconducting themselves in some other way.
That wordy definition has long been a source of legal debate.
The next question is whether the action they committed was so bad that it was "an abuse of the public's trust".
Lastly, if the evidence has passed those three tests, police need to examine whether the person under investigation acted "without reasonable excuse or justification".
That final question is crucial.
It's a fundamental principle of criminal justice that someone suspected of wrongdoing is given an opportunity to put forward their side of the story - and that starts when the police come and knock on their door.
Where has Mountbatten-Windsor been taken and what happens next?
King Charles's brother spent much of his 66th birthday in custody and we've just received word on where he had been taken.
The former prince was pictured slumped back in a vehicle following his release from Aylsham police station on Thursday evening.
Typically with arrests relating to white collar crime, people are held for a few hours to allow for searches and initial questioning.
Being released under investigation does not rule out further questioning at a later date.
At some point a decision will have to be made by the Crown Prosecution Service and Thames Valley Police about whether to charge the King's brother with misconduct in a public office.
It could be weeks before any substantial decision is made.
Where were the searches taking place?
On Thursday morning, Thames Valley Police said that, in addition to the arrest, it was "carrying out searches at addresses in Berkshire and Norfolk".
Unmarked police vehicles were seen arriving at the Sandringham Estate in Norfolk before Mountbatten-Windsor's arrest.
In an updated statement following Andrew's release, Thames Valley Police said its searches in Norfolk "have now concluded".
In recent weeks, Andrew has set up a temporary home on the estate whilst his permanent residence there, Marsh Farm, is being renovated.
Meanwhile, BBC News correspondent Helena Wilkinson, said she had seen uniformed police officers close to the Royal Lodge in Windsor Great Park.
Until very recently, this sprawling property in Berkshire is where Andrew lived.
BBC News understands that searches are being carried out at Royal Lodge and that the officers are there to guard the scene.
It is thought to be the only address being searched at the moment in Berkshire.
Dal Babu, a former Metropolitan Police chief superintendent, told BBC News that by arresting the King's brother, officers will be able to "to access computer equipment, files, photographs, any other evidence".
He said they will be able to "carry out searches of any premises he owns or occupies, or any other premises he controls, so there may well be searches in other areas as well".
What has the King said about the arrest? "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. "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