Police returned to the former home of Andrew Mountbatten-Windsor to continue searching for evidence a day after the former prince was arrested.
The brother of King Charles III was taken into custody on his 66th birthday yesterday on suspicion of misconduct in public office.
He was arrested at his new home at the King's Sandringham estate in Norfolk on the east coast of the UK, where he returned on Thursday night after about 11 hours in police custody.
AS IT HAPPENED: Andrew Mountbatten-Windsor leaves police station after being arrested
Officers maintained a presence at the house in Norfolk, about 185 kilometres north of London, on Friday, whilst detectives continued to search at Mountbatten-Windsor's former home at Royal Lodge in Windsor.
He has long denied any wrongdoing in relation to his friendship with convicted sex offender Jeffrey Epstein.
Police previously said they were "assessing" reports that Mountbatten-Windsor sent trade information to Epstein, a wealthy investor and convicted sex offender, in 2010, when the former prince was Britain's special envoy for international trade.
Correspondence between the two men was released by the US Justice Department late last month along with millions of pages of documents from the American investigation into Epstein.
"Following a thorough assessment, we have now opened an investigation into this allegation of misconduct in public office,'' Assistant Chief Constable Oliver Wright said in a statement.
Mountbatten-Windsor's arrest was not in relation to Epstein's abuse of girls.
READ MORE: What is 'misconduct in public office', the offence Andrew Mountbatten-Windsor has been arrested over?
Senior members of Britain's royal family continued to work on Thursday whilst Mountbatten-Windsor was in custody.
The King issued a statement soon after his brother's arrest, declaring his support for police to carry out their investigation.
"Let me state clearly: the law must take its course,'' the King said.
"As this process continues, it would not be right for me to comment further on this matter.''
The scrutiny of Mountbatten-Windsor has grown in recent years as the extent of his close friendship with Epstein was gradually revealed.
He was formally removed of all of his royal styles and titles late last year and forced to move out of Windsor Castle after it was revealed he had lied about cutting off his relationship with Epstein after the paedophile financier was first jailed for abuse charges.
READ MORE: Could Andrew Mountbatten-Windsor face charges? An expert says the process is difficult
Arrest was sudden, investigation will take time
Police swept into the grounds of Mountbatten-Windsor's home to arrest him at 8am on Thursday (7pm AEDT) — his 66th birthday — before taking him to Aylsham police station for questioning.
It's not known what he told them. He may have said nothing, or "no comment", as is his right.
Experts said that misconduct in a public office is notoriously difficult to prove.
"Firstly, it must be determined if Andrew Mountbatten-Windsor was in a role within government that constitutes the title of public officer," said Sean Caulfield, a criminal defense lawyer at Hodge Jones and Allen.
"There is no standard definition to clearly draw on."
The Crown Prosecution Service will ultimately make a decision about charging Mountbatten-Windsor, who remains eighth in line to the throne.
Andrew Gilmore, a partner at Grosvenor Law, said that prosecutors will apply the two-stage test known as the "Code for Crown Prosecutors".
"That test is to determine whether there is a more realistic prospect of a conviction than not based on the evidence and whether the matter is in the public interest," he said.
"If these two tests are met, then the matter will be charged and proceed to court."
A 'spectacular fall from grace'
"This is the most spectacular fall from grace for a member of the royal family in modern times," said Craig Prescott, a royal expert at Royal Holloway, University of London, who compared it in severity to the crisis sparked by Edward VIII's abdication to marry American divorcee Wallis Simpson.
"And it may not be over yet,'' Prescott added.
Thursday's arrest came a day after the National Police Chiefs' Council said it had created a group to assist forces across the Uk that are assessing whether Epstein and his associates committed crimes in Britain.
In addition to the concerns about Mountbatten-Windsor's correspondence, documents released by the US suggest Epstein may have used his private jet to traffic women to and from Britain.
– Reported with Associated Press
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