The gravest threat yet to Sir Keir Starmer’s premiership will crystallise on Tuesday when the House of Commons votes on whether to refer him to the Committee of Privileges over the aborted appointment of Lord Mandelson as ambassador to Washington. Speaker Lindsay Hoyle has granted a request from the Conservative leader, Kemi Badenoch, for a debate and binding vote on an investigation into whether the prime minister misled Parliament when he insisted that “full due process” had been followed. The outcome, although likely to be deflected by Labour’s substantial majority, exposes a vulnerability that the opposition will relentlessly exploit.

The immediate spark is the testimony of Sir Philip Barton, the former senior Foreign Office official who directly contradicted the prime minister’s account. Barton told MPs he found the sequence of events deeply irregular, because the appointment was announced before security vetting had concluded. More damaging still, he revealed he had explicitly flagged concerns about Lord Mandelson’s association with the late convicted sex offender Jeffrey Epstein, describing the matter as a “toxic, hot potato”, and underscored that Sir Keir was made aware of the risks yet chose to proceed. Viewed from London’s Westminster village, this contradiction transforms a political embarrassment into a potential breach of the ministerial code, sharpening the question of what the prime minister knew and when.

The pressure has been compounded by a striking act of self-incrimination from inside Sir Keir’s own circle. Morgan McSweeney, his former chief of staff and a figure long regarded as a protégé of Lord Mandelson, told the Foreign Affairs Committee that he had advised the prime minister in favour of the appointment but now conceded that was a “serious error of judgement”. McSweeney insisted no rules were deliberately bypassed, yet his admission that he “got it wrong” feeds the narrative of a dysfunctional decision-making culture at the heart of Number 10. Analysts in London note that McSweeney’s testimony, while stopping short of alleging dishonesty, dismantles the prime minister’s defence that standard procedures were observed.

From a wider geographic sweep, the affair reverberates far beyond Westminster. Viewed from Washington, the episode undermines the credibility of the United Kingdom’s most vital diplomatic posting just as the transatlantic alliance navigates trade frictions and the unpredictable return of Donald Trump. The notion that the occupant of the ambassador’s residence in Massachusetts Avenue might arrive trailing an Epstein cloud has caused quiet consternation among seasoned American diplomats, who prize discretion and unblemished judgement. In Paris, where Le Figaro has tracked the unfolding scandal with Gallic fascination, the sense is that Sir Keir’s authority on the European stage may suffer collateral damage; a prime minister embroiled in a sleaze inquiry at home struggles to project the stability he so often demands of others.

Crucially, Speaker Hoyle’s decision to permit the vote ensures that every Labour MP must go on the record, either shielding the prime minister or conceding that doubts are legitimate. Should the motion fall, as arithmetic suggests it will, the immediate legal jeopardy will recede but the political stain will not. A referral to the Committee of Privileges, however improbable, would be catastrophic: if it were to conclude that Sir Keir knowingly misled the House, the precedent of previous resignations on such grounds would make his position untenable. Even without a referral, the prime minister now presides over a party whose internal morale is sapped, and the opposition has been gifted a simple, resonant charge — that a government elected on a promise to clean up politics is itself cavalier with the truth. The long arc of this scandal, whatever the vote result, points towards a protracted erosion of trust that no whipping operation can fully repair.