Andy Burnham's path to becoming prime minister

Labour MPs are about to start nominating their next leader following Sir Keir Starmer's resignation.

Andy Burnham is so far the only MP who has announced their intention to stand, and is widely expected to become the prime minister later this month.

It means the former Greater Manchester mayor could enter Downing Street just a few weeks after he returned to Parliament in a by-election in Makerfield.

There will be a week-long window between 9 and 15 July for MPs to gather the required nominations to stand under the party's rulebook.

Leadership candidates need 20% of the party's MPs to nominate them. There are 403 Labour MPs, meaning the support of 81 is needed.

They also need to be nominated by at least three of the 31 socialist societies and trade unions affiliated to the party, or 5% of the constituency Labour parties (CLPs).

Affiliate nominees must comprise at least 5% of Labour's affiliated membership and include two unions, a group that includes GMB, Unite and Unison.

On 13 July, candidates will take part in a parliamentary hustings where they can answer questions from fellow Labour MPs. If Burnham remains the only candidate, he could in effect hold a hustings all by himself.

Affiliate nominations open at 18:00 BST on 15 July and close the same time the following day. The CLP nomination stage, if required, would begin on 20 July and end on 31 July.

This is the most likely scenario - so far, no other candidate has emerged as a likely challenger and most MPs are working on the assumption Burnham will become the next prime minister.

Wes Streeting, Sir Keir's former health secretary, had been seen as the MP most likely to throw his hat in the ring.

But that changed shortly after Sir Keir's resignation, when he instead endorsed Burnham and indicated he would not fight for the leadership.

If Burnham secured 323 nominations, it would be mathematically impossible for a rival to reach the 81-MP threshold needed to run against him.

If no other MP emerges as a challenger, Burnham could be the Labour leader by 17 July. However, he would not become the prime minister until three days later.

That's because Sir Keir Starmer would need to formally tender his resignation to King Charles III at Buckingham Palace. The King would then invite Burnham to form a new government.

As the leadership declaration would take place on a Friday, this official handover wouldn't happen until the next working day - Monday 20 July.

If another candidate does emerge, then the Labour leadership contest would take place over Parliament's summer recess from 16 July onwards.

Sir Keir Starmer would remain as the prime minister until Parliament returns on 1 September.

In this scenario, a ballot would take place among party members and affiliated trade union supporters between 6 and 27 August to decide the winner.

The result of this contest would be announced on 29 August.

If he becomes PM, Burnham has already ruled out an early election, telling an online forum last week: "I'm going to work to the 2024 manifesto."

By September at the latest, the UK will have had seven prime ministers in the past decade.

Four prime ministers took over mid-term without having fought and won a general election: Theresa May in 2016, Boris Johnson in 2016, and Liz Truss and Rishi Sunak, both in 2022.

In each of these cases, there have been questions from opposition parties about their legitimacy in office.

But the next prime minister, like their predecessors, is under no obligation to call a general election before the scheduled end of the Parliament in 2029.

The current system sees the electorate vote for their MPs and a party, and those MPs and that party choose their leader.

The last general election was held on 4 July 2024, with Labour winning a landslide majority, meaning the next election must legally be held by August 2029, though a prime minister can choose to call one at any point before this.

A PM calling an early election depends on a number of factors - how the party is performing in the polls, and whether the prime minister wants to seek a new mandate from the electorate to set out his own agenda in government.

So while Burnham has ruled out an immediate election, there may still be one before the end of 2029.