Labour officially started choosing its next leader on Thursday morning. MPs can now officially support candidates to replace Sir Keir Starmer. So far, only Andy Burnham has said he wants the job.

Here's how Labour's rules work. Any candidate needs support from at least 81 MPs. They also need backing from either 32 local party branches, or three affiliated organisations like trade unions. Candidates have until next Wednesday to reach the MP target. Labour currently has 326 MPs.

Here's where the maths gets interesting. If Burnham gets 323 MP nominations, nobody else will be able to gather the 81 they need to even get on the ballot. That would mean the contest is over before ordinary party members get to vote. It would all be decided by MPs alone.

Other potential candidates have stepped aside. Wes Streeting, the former health secretary and the main rival, ruled himself out on 22 June. He has now publicly backed Burnham. Another potential challenger, Al Carns, dropped out just before nominations opened on Wednesday evening. The field has quietly narrowed instead of becoming crowded.

Burnham returned to Parliament very recently. He had spent ten years as mayor of Greater Manchester, a role outside Westminster. He came back through a by-election on 19 June. Within days, Starmer announced he was stepping down as Labour leader, saying he was no longer the right person to lead the party.

Burnham has stressed he plans to continue the government's current economic approach. On 29 June, he committed to keeping the existing fiscal rules — the plan for how the government handles money — in place. When asked about calling an early general election, he said it was too soon to discuss.

Labour's rules are supposed to let party members and trade unionists have a say in choosing their leader. But here's the tension. Those same rules allow a contest to end if nobody can gather enough MP support to challenge the frontrunner. If that happens, party members would have nothing to vote on. If Burnham reaches 323 nominations, he would be declared leader by the end of the nomination week and would become prime minister on 20 July — with the wider party having had no say.