King Charles' younger brother Andrew Mountbatten-Windsor was released from police custody on Thursday evening after being arrested on suspicion of misconduct in public office over allegations he sent confidential government documents to Jeffrey Epstein.

Mountbatten-Windsor, who turned 66 on Thursday, had been questioned all day by detectives from Thames Valley Police. Earlier this month, the police force said it was looking into allegations that he had passed documents to the late convicted sex offender while working as a trade envoy.

The arrest of the senior royal, eighth in line to the throne, is unprecedented in modern times.

"I have learned with the deepest concern the news about Andrew Mountbatten-Windsor and suspicion of misconduct in public office," King Charles said in a statement.

A Reuters witness saw the former prince leaving a police station in Aylsham, eastern England, where he was met by a small group of photographers and television crews, just after 1900 GMT.

A Reuters photograph taken after his release shows him seated inside a car, appearing visibly shaken.

Thames Valley Police said later on Thursday that "the arrested man" had been "released under investigation."

'The law must take its course'

Although Buckingham Palace was not informed in advance about the arrest, Charles said the authorities had the family's "full and wholehearted support and cooperation".

"Let me state clearly: the law must take its course," the monarch said in his statement.

"Meanwhile, my family and I will continue in our duty and service to you all."

The king visited a fashion show in London on Thursday, making no further public comment.

Mountbatten-Windsor, the second son of the late Queen Elizabeth, has always denied any wrongdoing in relation to Epstein, and said he regrets their friendship.

His office did not respond to a request for comment and he has not spoken publicly since the release of more than 3 million pages of documents by the US government relating to Epstein, who was convicted of soliciting prostitution from a minor in 2008.

Those files suggested Mountbatten-Windsor had in 2010 forwarded to Epstein reports about Vietnam, Singapore and other places he had visited on official trips as the government's Special Representative for Trade and Investment.

He was forced to step down from that role in 2011 after his close links to Epstein first emerged.

"Following a thorough assessment, we have now opened an investigation into this allegation of misconduct in public office," Thames Valley's Assistant Chief Constable Oliver Wright said in a statement.

"We understand the significant public interest in this case, and we will provide updates at the appropriate time."

The arrest marks a new low for the former prince, who was forced to quit all official royal duties in 2019 over his ties with Epstein and was then stripped by his older brother of his titles and honours last October amid further revelations about their relationship.

Unmarked police cars

Earlier, six unmarked police cars and around eight plain-clothed officers were pictured at Wood Farm on the Sandringham estate in eastern England where Mountbatten-Windsor now resides.

Thames Valley Police officers were also searching the mansion on the king's Windsor estate where Mountbatten-Windsor lived until he was forced out amid anger at the Epstein revelations.

While being arrested means that police have reasonable suspicion that a crime has been committed and that the royal is suspected of involvement in an offence, it does not imply guilt.

A conviction for misconduct in a public office carries a maximum sentence of life imprisonment, and cases must be dealt with in a Crown Court, which deal with the most serious criminal offences.

Police have previously said misconduct in public office, which is a 'Common Law' offence and is not covered by written statute legislation, involved "particular complexities".

Giuffre lawsuit

In 2022, the king's brother settled a civil lawsuit brought in the United States by the late Virginia Giuffre who accused him of sexually abusing her when she was a teenager at properties owned by Epstein or his associates.

The current police investigation is not related to this or any other allegation of sexual impropriety.

"Today, our broken hearts have been lifted at the news that no one is above the law, not even royalty," the family of Giuffre, who died by suicide last year, said in a statement.

In response to Mountbatten-Windsor's earlier arrest, US President Donald Trump said it was a "shame."

"I think it's very sad. I think it's so bad for the royal family," Trump told reporters. "It's very, very sad to me... to see what's going on with his (King Charles') brother."

Were Mountbatten-Windsor to ultimately face criminal charges, he would join a very small group of senior British royals who have formally been accused of offences.

His elder sister Princess Anne was fined for speeding in 2001, and the following year became the first royal to be convicted of a criminal offence in 350 years when she appeared in court to plead guilty to failing to stop one of her dogs, named Dotty, biting two children.

King Charles I was tried for treason in 1649 towards the end of the English Civil War, found guilty and beheaded.

The misconduct investigation is not the only accusation against Mountbatten-Windsor into which police are looking.

Anti-monarchy campaign group Republic has reported him over allegations he was involved in the trafficking of a woman to Britain for sex in 2010. Thames Valley Police said it was assessing allegations that a woman had been taken to an address in Windsor, where the former prince lived until recently.

US lawmakers have also said he should testify before committees in the United States over what he knew about Epstein.

Meanwhile, former British Prime Minister Gordon Brown has also called for a police investigation into the extent of Epstein trafficking women without proper checks by the authorities through London's Stansted Airport, saying this had been overlooked by previous inquiries into Mountbatten-Windsor.

Essex Police said on Wednesday it was looking into the issue.