PA MediaPolice are investigating a 2010 email Lord Mandelson appeared to share with convicted sex offender Jeffrey Epstein about an EU bailout after the financial crash, Met chief Sir Mark Rowley has said.
In an interview with ABC News in the US, the head of the Metropolitan Police said officers were looking at whether this was a criminal offence.
The UK's former ambassador to the US is under investigation over allegations of misconduct in public office.
He has repeatedly let it be known that he believes he has not acted criminally, did not act for personal gain and is co-operating with the police.
One email released by the US Justice Department as part of the Epstein files earlier this year suggests Lord Mandelson gave the financier advance notice of a €500bn bailout by the EU to save the ailing Euro.
The evening before the announcement on 10 May 2010, Epstein emailed Lord Mandelson to say: "sources tell me 500 b euro bailout , almost complete."
The documents appear to show Lord Mandelson replied: "Sd be announced tonight."
Epstein then asked if Mandelson was home and received a reply, saying: "Just leaving No10..will call."
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Sir Mark told ABC News: "As we've said publicly with Peter Mandelson, the former ambassador, there is a particular e-mail to do with bailouts after the financial crash in the sort of, I think 2008-2009.
"It looks like it was shared with Epstein, so we're looking at that as to whether that's a criminal offence."
He added that Thames Valley Police were also looking at other documents potentially shared by Andrew Mountbatten-Windsor.
The former prince is also under investigation on suspicion of misconduct in public office.
He was arrested after the force said it was assessing a complaint over the alleged sharing of confidential material with Epstein.
Sir Mark said there were also "a whole range of suggested sexual allegations" against Mountbatten-Windsor "and those are being assessed at the moment to see whether any of them do actually merit a criminal investigation".
The Met chief said officers recorded four interviews with Virginia Giuffre, who alleged that she was brought to London to have sex with the former prince in 2001 when she was 17.
He added: "Those interviews didn't give us any evidence or any allegations of sexual offending or trafficking that we could investigate in the UK.
"So that's why that investigation didn't go forward."
Mountbatten-Windsor has always denied the allegations.
Giuffre died by suicide last year.
Sir Mark said he was in the US because there was "a big body of evidence" in the Epstein files "and we're going to need the unredacted evidence" if any cases reach court.
Asked if the US was being cooperative, Sir Mark said: "Yeah, so those conversations are where I'd expect them to be at the moment."

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Jeffrey Epstein
