Andy Burnham, the mayor of Greater Manchester, won a parliamentary seat on Friday, a vital step towards fulfilling his ambition to replace his party colleague Keir Starmer as Labour leader and Britain’s prime minister.

About a quarter of Starmer’s lawmakers have urged the prime minister to resign since his Labour Party last month suffered the heaviest local election losses of any governing party in more than three decades.

Burnham has said he would run in any leadership race, and is the frontrunner to replace Starmer. But some of his supporters are divided over how, and when, to challenge the prime minister.

Below are some of the ways that Burnham could topple Starmer:

Burnham triggers early contest to challenge Starmer

Burnham will be sworn into parliament next week, giving him a chance to formally challenge Starmer, as only members of parliament can be prime minister.

Labour has strict rules governing the removal of a party leader. The system requires lawmakers to coalesce around specific candidates rather than just express “no confidence” in their current leader.

Under the rules, any challenger must secure the backing of 20 per cent of the party’s lawmakers in the House of Commons, equating to 81 lawmakers, including the challenger, in the current lower house of parliament.

If Burnham decides to move early, he will most likely need to announce a formal challenge before parliament goes into recess on July 16, when lawmakers return to the areas they represent to work and take summer holidays.

Starmer has an automatic right to stand in any leadership contest, and has said he would do so.

After candidates gather support from local party branches and trade unions, party members would then vote.

Overall, that could take two to three months to complete.

Burnham waits until after the summer to trigger a contest

Burnham could seek to delay any leadership bid until after the summer, to give him time to campaign for the Labour candidate to replace him as the Greater Manchester mayor, in an election expected to take place around the end of July.

It would also allow Burnham the opportunity to build relationships and seek more support from Labour members of parliament, particularly those elected since he left parliament in 2017.

Another politician triggers a leadership contest

A leadership contest could also be triggered by another politician, with Burnham then choosing to stand.

Wes Streeting, who resigned as health minister in May, has said Starmer should consider his position over the weekend. If the prime minister fails to set out the terms of his departure, Streeting says he is prepared to challenge him soon.

If Starmer and Streeting stand against Burnham in a vote among Labour members, polls suggest Burnham would win.

Burnham victory triggers an orderly transition of power

In another scenario, Burnham could be the beneficiary of an uncontested handover of power.

Many Labour politicians have urged Starmer to set out a timetable for his departure rather than awaiting and then contesting a leadership election.

A one-on-one contest between Burnham and Starmer could damage the party if both politicians engage in personal attacks.

If Starmer chose not to stand, and Burnham secured enough backing from members of parliament to enter the race while his rivals failed to do so, Burnham would become prime minister.

Lawmakers have suggested Streeting might also accept a senior role in a Burnham government, such as finance minister, in return for not triggering a leadership contest.