Nominations for the Labour leadership contest opened at 09:00 BST on Thursday, with MPs able to formally endorse candidates to replace Sir Keir Starmer. Andy Burnham is, so far, the only name in the field BBC. Labour said it would publish an initial tally of nominations at around 19:00 BST the same day.

The mechanics matter as much as the politics. Labour's rulebook requires any candidate to secure nominations from 81 MPs, plus either backing from 32 of the party's 634 constituency branches or from three of its 31 affiliated socialist societies and trade unions, at least two of which must be unions BBC. Candidates have until the following Wednesday to hit the MP threshold. Given Labour's current parliamentary arithmetic, if Burnham reaches 323 nominations it becomes mathematically impossible for any rival to assemble the 81 required to get onto the ballot at all — a threshold that would settle the contest before members ever cast a vote.

That prospect is not new. The Guardian reported as early as 20 June that Burnham's allies were confident of an unopposed "coronation", following a surge in declared MP support The Guardian. The same report noted that some Labour MPs were uneasy at the idea of a contest closing down before it began, preferring Burnham to win a mandate through an actual vote rather than by mathematical foreclosure. Those reservations do not appear to have altered the trajectory of events.

The field has narrowed steadily rather than dramatically. Wes Streeting, the former health secretary and the most obvious potential rival, ruled himself out on 22 June, a decision The Guardian said left Burnham the overwhelming favourite to succeed Starmer as leader and prime minister as soon as July The Guardian. Streeting has since gone further and thrown his support behind Burnham directly, one of several potential challengers to do so. Al Carns, the former defence minister who had floated a run, ruled himself out on Wednesday evening, hours before nominations opened BBC.

It is worth recalling how unlikely this contest would have looked a matter of weeks ago. Burnham had been out of Parliament for a decade, having left the Commons for Greater Manchester's mayoralty, a devolved office with no seat at Westminster. He returned via the Makerfield by-election on 19 June, a result Reuters described as decisive for Labour and one that immediately cleared his path to challenge Starmer Reuters. Starmer initially vowed to fight any leadership challenge that followed Reuters, but announced his resignation within days, telling the party in an emotional statement around 22 June that he had listened to Labour and concluded he was no longer the right person to lead it Reuters.

Burnham has used the intervening weeks to signal continuity rather than rupture on economic policy, committing on 29 June to keep the government's existing fiscal rules in place Reuters. Asked in late June about the prospect of an early general election, he said it was too soon to discuss one Reuters — a formulation that leaves the door notionally open without committing him to anything.

The party's internal rulebook has been a live issue since well before Burnham's return to the Commons. The Guardian reported on 14 May that his prospective backers were already lobbying the National Executive Committee over the contest's rules, and that one NEC member warned separately against what they called a "Wes Streeting coronation" The Guardian. The irony, several weeks on, is that the coronation risk has simply changed names.

If no rival candidate qualifies, Burnham will be declared Labour leader at a party event at the end of the nomination week, and would become prime minister on 20 July. Under that scenario, Labour members and affiliated trade union supporters — the electorate the party's rules were designed around — would get no vote at all. That outcome would not breach the letter of Labour's rulebook, which permits unopposed nominations to stand. But it would be a striking way for a Labour prime minister to take office: chosen by 81 or more colleagues in the parliamentary party, rather than by the wider movement the leadership contest nominally exists to consult.