Prosecutors are "providing early investigative advice" to the police as they carry out inquiries into Peter Mandelson and Andrew Mountbatten-Windsor's links to convicted paedophile Jeffrey Epstein.
Both Mandelson, the former UK ambassador to the US, and Andrew, the former Duke of York, were arrested in February on suspicion of misconduct in public office over their connections to Epstein.
Both Mandelson and Andrew were released under investigation.
"We can confirm that we are now providing early investigative advice to the Metropolitan Police Service regarding the force's investigation into a 72-year-old man and will continue to liaise as requested," a Crown Prosecution Service (CPS) spokesperson said on Wednesday.
"We can now also confirm that we are providing early investigative advice to Thames Valley Police in relation to a 66-year-old man.
"We remain ready to support any investigation if needed."
Andrew was arrested at Sandringham on 19 February on suspicion he shared sensitive information with Epstein when he was UK trade envoy, a role he served in between 2001 and 2011.
Andrew has repeatedly denied any wrongdoing in connection with Epstein.
Lord Mandelson was arrested on 23 February on suspicion of misconduct in public office. The Metropolitan Police is looking into allegations he passed sensitive information to Epstein during his time as business secretary, a role he served from 2008 to 2010, when Gordon Brown was prime minister.
He has denied any wrongdoing.
Epstein died by suicide while awaiting trial on sex trafficking charges in 2019.
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It comes as the King's state visit to the US was confirmed.
Buckingham Palace revealed on Tuesday that a planned visit this month by Charles and Queen Camilla would go ahead despite an ongoing conflict in the Middle East, which has strained transatlantic relations.
It is understood that Charles and Camilla will not be able to meet with Epstein's victims while there are ongoing UK police investigations into matters related to the paedophile financier.
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