Those who don’t pay much attention to politics could be forgiven for not being fully on top of this, but the UK could have a new prime minister soon – potentially within weeks.

So how quickly could Andy Burnham replace Keir Starmer, if Burnham wins Thursday’s Makerfield byelection? Actually, quite quickly, but only if events go in a particular way. And there are plenty of alternate outcomes.

The fastest: Burnham in No 10 within days

The Labour party rulebook sets out that if an MP wants to challenge a sitting party leader, they must first be nominated by at least 20% of the parliamentary party (currently 81 MPs), as well as by at least 5% of all local branches or at least three party-affiliated groups, of which a minimum of two must be unions. Any contenders who meet the threshold go forward to a vote of party members, assuming there is more than one challenger.

It also says that if a sitting leader is challenged, they do not need to meet these requirements and are automatically in the members’ vote – if they choose to do so.

There is a scenario in which Burnham is elected as Makerfield’s MP, Starmer decides he will step down immediately – perhaps after a mass of senior cabinet ministers tell him they will resign if he does not – and no other challengers come forward.

Burnham would still need to go through the formal process of getting support from MPs and party groups, but with a coronation looming this would most likely be swift. Labour’s ruling national executive committee (NEC) would set a timetable for the contest, and without a vote of the membership, the process could be completed in days.

Slightly slower: Burnham in No 10 within weeks

Keir Starmer has said he will fight any leadership challenge. Photograph: Isabel Infantes/AFP/Getty Images

This is largely a re-run of the scenario above, but with the difference that Starmer agrees a timetable to depart which takes slightly longer, perhaps including a final diplomatic mission to the Nato summit in Ankara in early July.

Without a full contest taking place, Burnham and his advisers might be sanguine about this, within reason – he would most likely wish to take over before parliament goes into recess later in July.

It is worth noting at this point that Starmer has repeatedly ruled out stepping aside, saying he would fight any contest. This resolve might, however, be swayed by either very obvious mass support for Burnham among Labour MPs, or large numbers of cabinet ministers threatening mutiny.

It is also contingent on no other challengers coming forward, most notably Wes Streeting. The health secretary has said he will launch a challenge and has enough MP backers, even if the latter remains to be seen.

Quite a bit slower: a full contest

If Starmer, Streeting or anyone else decides to fight Burnham for the leadership, and they get the sufficient suite of nominations, it will go to a vote of Labour’s membership. The timetable will necessarily be longer, but how much longer depends.

The timetable is set by the NEC, which has some leeway. In 2020, when Labour was in opposition and had just lost the December 2019 election, it set aside three months for the contest which saw Starmer take over as leader.

But in September 2025, with Labour in government and Angela Rayner having resigned as deputy leader, the contest to find her replacement – held under the identical rules – was accelerated, taking about six and a half weeks.

Choosing a new prime minister is, of course, even more urgent, and so the timetable could be compressed more, although Labour officials will want to ensure there is time for candidates to get a proper hearing.