The Cabinet Office is set to release the second tranche of the "Mandelson files," comprising over 1,000 pages of documents pertaining to Peter Mandelson's controversial appointment as UK ambassador to Washington. This substantial disclosure marks the second-largest document dump to the House of Commons since the Chilcot inquiry and is anticipated to revive uncomfortable questions regarding Prime Minister Keir Starmer's judgment in bestowing such a critical diplomatic role upon Mandelson. The extensive collection includes WhatsApp messages and emails from ministers, officials, and advisers, many of which are expected to prove embarrassing and potentially further destabilize Starmer's embattled government. While the release, compelled by a humble address process, will exclude information detrimental to UK national security or international relations, a rigorous redaction process was carried out by the intelligence and security committee, and some documents are being withheld at the Metropolitan Police's request for potential future prosecution.

Among the potentially damaging disclosures are exchanges from Mandelson's tenure in Washington, which could feature criticism of the prime minister and awkward WhatsApp messages from ministers "trying to impress" the former US ambassador. These might include group messages involving former health secretary Wes Streeting, who has reportedly sought to distance himself from Mandelson. Government insiders have also suggested that the documents will reveal Mandelson offering unsolicited advice to ministers on policy matters beyond his official diplomatic brief, further questioning the scope and nature of his influence.

A central point of contention revolves around what has been deliberately omitted from the document tranche, which many believe is as significant as what is included. Notably absent is a nine-page summary document compiled by UK Security Vetting (UKSV). The Guardian previously disclosed that UKSV had raised serious concerns about Mandelson's associations with senior figures in China, Russia, and Israel, concluding that he should be denied security clearance – details all contained within this summary. However, officials decided against releasing this crucial document after the Metropolitan Police requested its suppression. This omission directly impacts the transparency surrounding the security vetting process for such a high-profile appointment.

Compounding these concerns is the startling revelation that the documents contain no written record of any measures taken to mitigate the serious security risks identified by UKSV concerning Mandelson's appointment. Multiple sources, including those briefed on the files, confirm the absence of details about steps put in place to address these "red flags." This directly contradicts assurances given to MPs by senior Whitehall officials on the foreign affairs select committee, who had been told that Mandelson agreed to undertake specific steps to allay concerns. While management actions were taken to address commercial conflicts of interest related to Mandelson's stake in his co-founded lobbying firm, Global Counsel, these were explicitly separate from national security mitigations. Former head of MI6 sources suggest that the breadth of risks identified by UKSV would have rendered effective security mitigations "totally impossible."

The controversy is further fuelled by conflicting accounts from senior officials. Olly Robbins, the senior government official who was sacked for his role in Mandelson's appointment, previously told MPs that UKSV considered Mandelson a "borderline case" and was only "leaning towards" recommending denial of clearance. However, no published documents thus far support this "borderline" claim, and the term has not been used by others with access to UKSV documents; in fact, a UKSV decision template reportedly suggests the opposite. Robbins, alongside Foreign Office security chief Ian Collard, initially approved Mandelson's clearance in late January 2025 based solely on oral briefings, without reviewing the critical summary document. It was not until seven months later, when Mandelson was withdrawn from Washington, that they decided to review the summary. The exact trigger for this belated review and the contents of the notes taken remain unclear, despite explanations given by Robbins and the Cabinet Office's top civil servant regarding requests for the document.

The political fallout from Mandelson's appointment has been significant for Keir Starmer, leading to the departure of his chief of staff, Morgan McSweeney, and Olly Robbins himself. The lack of documented security mitigations is poised to raise significant questions among MPs, particularly given explicit assurances that such measures had been taken. Despite claims by Robbins, Cat Little (the top official gathering humble address material), and Ian Collard about emails recording the decision and mitigations, multiple sources indicate that these emails lacked any formal agreement with Mandelson, implying any such arrangements were informal at best. The imminent release of these documents promises another challenging period for Starmer's government, underscoring the ongoing scrutiny over a decision that has already led to high-profile resignations and undermined prime ministerial authority.