In brief – what’s happening and why does it matter?

Voters in Makerfield, a largely suburban constituency on the edge of Wigan, Greater Manchester, are choosing a new MP after the incumbent stood down. The candidate for the ruling Labour party and favourite to win, Andy Burnham, is the mayor of Greater Manchester and a former MP.

If Burnham wins, it is widely assumed that he will launch a campaign to unseat Keir Starmer as Labour leader and thus UK prime minister, or persuade Starmer to stand down. If things go as expected, the UK could have its seventh prime minister since 2017 in just a few weeks or months.

How does all this work constitutionally?

The prime minister is, under very strong constitutional convention – there is no written constitution in the UK – always a member of the House of Commons, all 650 of whom represent a specific geographical constituency.

Burnham was from 1997 to 2017 MP for the adjoining seat of Leigh, serving as a government minister and twice standing unsuccessfully for the Labour leadership. But he left Westminster when Labour was in opposition after being elected Greater Manchester mayor.

For him to challenge Starmer, Burnham needs a new seat. After much wrangling one of his allies, Josh Simons, agreed to stand down as the Makerfield MP, triggering what is known as a byelection, a single-seat contest held when there is a vacancy between general elections.

Why does Burnham want Starmer to go?

It’s not just him. About 100 Labour MPs have publicly called on Starmer to quit, with many others privately sharing the sentiment. A lot of it is self-interest – Labour is consistently polling well behind the hard-right populist party Reform UK, and performed terribly in May in elections for the devolved Scottish and Welsh parliaments and local government seats across England.

But many Labour MPs also think Starmer lacks any political vision or much of a plan, and have seen him lurch from U-turn to U-turn. A number are worried that without action, Reform will win the next election.

Will Burnham win the seat?

The short answer is that, yes, most people believe he will, even if it could be a fairly close result. The longer answer is that Makerfield is the sort of seat Labour used to win with ease – the party has held the constituency since it was created in a 1983 reorganisation of parliamentary boundaries – but now struggles with.

With its voters disproportionately being older, less well educated and less likely to come from minority ethnic backgrounds, Makerfield, like dozens of similar post-industrial seats in the north of England, now heavily favours Reform, led by Nigel Farage, the driving force for the UK’s decision in 2016 to leave the EU.

Burnham’s hope is that his charisma, plus his reputation as a generally popular mayor, will get him over the line. He has also been helped by Reform’s slightly misfiring campaign, with the party’s candidate, a local plumber called Robert Kenyon, criticised about past social media posts, including some sexist and otherwise lewd comments.

If Burnham wins, what happens next?

Starmer has repeatedly said that, just two years from a big general election win, he is not going anywhere and will fight any challengers. Nonetheless, Burnham’s allies hope he can be persuaded to set a timetable to leave.

If that happened and no other MP sought the leadership, Burnham could be made prime minister within weeks. But at least one other MP, Wes Streeting, who resigned as health secretary last month, has promised to enter the race.

If there is a contest, how does it work?

An MP needs at least 80 nominations from other Labour MPs to trigger a contest. Any other MP who reaches the same threshold also takes part, as does the sitting leader, if they choose to, by default.

Those names are then put to a vote of Labour party members, a process that would take four or five weeks at least, given the need for hustings events and similar.

Who would win such a contest?

Again, the money is largely on Burnham, although some supporters of Starmer argue he could appeal to the loyalty of Labour members who chose him in 2020.

Streeting is seen as an outsider but is a talented political operator, who is considered to be on the centre-right of the Labour party.

What would a Burnham government look like?

Some cynics say: much like Starmer’s, but with better presentation. And more seriously, many Burnham supporters enjoy his relaxed eloquence, particularly against Starmer’s often pained and stilted approach.

Policy-wise, while once on the centre-right of the party, Burnham has as a mayor developed a more left-leaning philosophy, based on a version of what is almost a European-style corporatism. Awkwardly known as “Manchesterism”, it sets out that while free markets are central, they must be tempered by some state control, even ownership, particularly in necessities of life such as utilities, transport and housing.