The gravest threat yet to Sir Keir Starmer’s leadership crystallises on Tuesday, when MPs will vote on whether to refer the prime minister to the Commons Privileges Committee over claims he misled Parliament about the vetting of Peter Mandelson. Speaker Sir Lindsay Hoyle has granted a debate and binding vote on the motion tabled by the Conservative opposition, raising the spectre of an investigation that, as Boris Johnson discovered, can end a political career. The immediate question is whether Starmer was sufficiently candid about what he knew and when concerning the now-disgraced peer, but the broader crisis has already exposed the fault line between the cautious Labour leader and the party machine that elevated him.

Viewed from Washington, where ambassadors matter immensely in the choreography of transatlantic trust, the affair risks destabilising a diplomatic posting still in its infancy. Mandelson’s appointment was never Starmer’s instinct; it was the brainchild of his then chief of staff Morgan McSweeney, who saw in the supremely networked and nimble Mandelson the ideal envoy to Donald Trump’s capricious court. The prime minister acquiesced, overruling his own well-documented aversion to the old Blairite guard.

That decision now haunts him after Mandelson was fired last September when his ties to the late sex offender Jeffrey Epstein proved deeper than previously acknowledged. In London, the parliamentary arithmetic is brutally simple. Labour’s large majority means the referral motion is almost certain to be voted down.

Yet analysts in the capital note that a hollow victory would not staunch the political bleeding. The very fact of the debate forces Starmer to rely on whipping his MPs into line on a question of personal integrity, a process that often leaves unseen scars. The prime minister has insisted the “vast majority” of his parliamentary party remains “supportive, loyal and just want to get on with the job”, but such declarations tend to invite scrutiny of the mutinous minority.

From Berlin, seasoned observers of British politics detect a familiar cycle. One German daily has already christened him “Boris Starmer”, an allusion to the irony of a leader who promised probity now fighting for his life on the same procedural terrain that consumed his predecessor. The comparison is inexact but corrosive: Johnson was felled by lockdown parties, Starmer by a personnel decision that strikes at his claim to have purged the grubby habits of the past.

In either case, the Privileges Committee has become the preferred instrument for resolving whether a prime minister’s word can still be trusted. Viewed from the Asia-Pacific, where Britain is striving to project post-Brexit reliability, the spectacle of another premier mired in a standards scandal weakens London’s diplomatic hand. Allies in the region quietly worry that a diminished Starmer will lack the authority to sustain the strategic coherence required by the AUKUS pact and broader Indo-Pacific engagement.

Even if he survives Tuesday’s vote, the prime minister’s room for manoeuvre on the domestic front will shrink. The Mandelson affair has revealed a leader caught between the instincts of his own caution and the ambitions of his backroom architects. That tension will not dissipate with a whipped majority; it will simply await the next moment when the interests of the Labour Party and the judgement of Sir Keir Starmer diverge.