Why did Britain's most senior civil servant fail to open an investigation last autumn? That's the question Gordon Brown is now asking as the Mandelson scandal deepens.
The revelation that Lord Mandelson shared classified government material with Jeffrey Epstein has prompted the Metropolitan Police to open a criminal investigation, announced Tuesday.
Mr Brown had flagged his concerns to Cabinet Secretary Sir Chris Wormald back in September - yet no inquiry followed, a decision the former prime minister is now challenging in a move widely interpreted as an attack on Prime Minister Kier Starmer, as intense scrutiny will now inevitably fall on Number 10 decision-making.
Gordon Brown and Peter Mandelson had one of the most toxic relationships in modern British politics. Their feud dates back to the 1990s when both were key figures in Tony Blair's New Labour project, with Mandelson widely seen as being in Blair's camp during the long-running Blair-Brown rivalry for party leadership and control of government direction.
The animosity deepened after Mandelson was forced to resign twice from ministerial positions - first in 1998 over an undisclosed loan, then again in 2001 over a passport application controversy - yet Brown had to bring him back as business secretary in 2008 during the financial crisis, creating an uneasy partnership where Mandelson effectively became Brown's deputy despite their mutual distrust.
It was announced on Tuesday that Mandelson had stepped down from the Lords. The correspondence in question with the sex offender dates back to his time running the business department under Mr Brown, when he also functioned as de facto deputy premier from 2009 to 2010.
This police probe means officers will be expected to interview prominent politicians from the New Labour era, a list likely to include Mr Brown himself alongside Lord Mandelson.
The scandal could consume Sir Keir Starmer's administration, as pressure mounts for him to explain why he pushed ahead with Lord Mandelson's Washington appointment even after being told about the Epstein connection.
The former PM has supplied what he terms "relevant" evidence to investigators while publicly attacking Lord Mandelson's conduct as an "inexcusable and unpatriotic act."
Mr Brown detailed his police submission: "I have included the letter I sent in September 2025 asking the Cabinet Secretary to investigate the veracity of information contained in the Epstein papers regarding the sale of assets arising from the banking collapse and communications about them between Lord Mandelson and Mr Epstein.
"I have also included the November 2026 [sic] response from the Cabinet Secretary who said about this that 'no records of information or correspondence from Lord Mandelson's mailbox' could be found.
"Having drawn their attention to relevant evidence, the matter now rests in the hands of the police."
The 2026 date looks like a mistake - November 2025 seems the correct reference.
What did the correspondence contain? Details of how the government planned to handle a massive EU rescue package worth billions, when Mr Brown intended to quit, and which risked public land and buildings being sold off.
Separate documents from American justice officials revealed a monthly $4,000 payment arrangement from Epstein to Lord Mandelson's spouse - established during the peer's time as number two in government.
Further messages between them explored tactics for destabilising both Mr Brown and his then-chancellor Alistair Darling as the 2010 vote approached.
While the exchanges predominantly involve New Labour veterans, key members of Sir Keir's current team are also facing scrutiny.
Downing Street's chief of staff Morgan McSweeney faces intensifying questions - he was the one who put Lord Mandelson's name forward for the ambassadorial role.
Tuesday's Cabinet session saw the Prime Minister unleash a blistering attack on Lord Mandelson while voicing concerns that additional disclosures may be coming.
The transmission of correspondence about "highly sensitive government business" was branded "disgraceful" by Sir Keir, who added he was "not reassured that the totality of the information had yet emerged."
A Downing Street spokesman reported: "The Prime Minister said Peter Mandelson had let his country down. He added the public don't really see individuals in this scandal, they see politicians.
"For the public to see politicians saying they can't recall receiving significant sums of money or not was just gobsmacking – causing them to lose faith in all politicians and weaken trust still further. The Prime Minister said that was why moving quickly in this matter was vital."
Why wasn't an investigation launched earlier? The PM's spokesman explained Mr Brown's letter to Sir Chris had focused narrowly on one matter - an RBS-linked venture disposal. The emails that seem to demonstrate Lord Mandelson sharing market-sensitive material with Epstein only surfaced in the past few days.
Expect more parliamentary pressure on Wednesday when the opposition deploys a humble address attempting to compel release of McSweeney-Mandelson communications.
The Telegraph reported on fresh information about the previously tight professional bond between the two men.
According to one source from the opening phase of Sir Keir's government, Downing Street staff knew frequent conversations and texts were happening between them.
Multiple trips by Lord Mandelson to Mr McSweeney's Scottish property occurred, while the pair synchronised their vacation schedules ahead of last year's election battle.
Lord Mandelson provided personal counsel to Mr McSweeney about September's Cabinet shake-up - prompted by Angela Rayner stepping down from her deputy PM role.
This means they were still in touch just seven days before Sir Keir dismissed Lord Mandelson over the Epstein correspondence that came to light following the reshuffle.
However, the guidance wasn't requested, a No 10 source countered - describing it as "unsolicited."
Labour MPs from the party's Left are pushing for Mr McSweeney's removal - he is widely seen as being responsibly for pushing their faction of the party to the margins. One told The Telegraph: "If he can't be removed in circumstances like this then he's never going to go."
Both David Lammy, the Deputy Prime Minister, and Pat McFadden, the Work and Pensions Secretary, served under Lord Mandelson when he ran the business department and may be questioned about their knowledge.
A Cabinet Office spokesman said: "Gordon Brown wrote to the Cabinet Secretary specifically asking about the involvement of Jeffery Epstein in the sale of Royal Bank of Scotland assets to JP Morgan and the potential involvement of Jeffrey Epstein. The letter did not ask for a wider investigation to take place.
"The Cabinet Secretary replied to the letter setting out that the searches of the officials records did not find any files linking Mr Epstein to the asset sale.
"The files released by the Department of Justice raise much wider questions about unauthorised disclosure of official documents from non-government systems. Following a request from the Prime Minister, the Cabinet Secretary is reviewing all available information regarding Peter Mandelson's contacts with Jeffrey Epstein during his period as a government minister."
Lord Mandelson has apologised to Epstein's victims, saying he had been "taken in" by a "charismatic criminal liar", but has not commented on newly emerged emails showing him passing on sensitive government information. He did not immediately comment on the launch of the Met investigation.
