What happens if Catherine West pulls the trigger on a leadership challenge?

Sir Keir Starmer is under huge pressure on the Labor leadership after his party suffered devastating election losses across the country.

A growing number of the prime minister’s own MPs, including some former loyalists, have called for him to go.

Speculation about the leadership battle had been mounting for months leading up to Thursday’s election, with Andy Burnham, Angela Rayner and Wes Streeting named as potential successors.

But on Saturday it was a more obscure backbencher who gave the cabinet an ultimatum to end Sir Keir’s premiership.

Labor MP Catherine West has told the cabinet that if they do not put forward a challenger to Sir Keir by Monday she will try to launch a leadership race herself.

The Prime Minister has insisted he will not resign, while cabinet minister Bridget Phillipson insisted on Sunday that Ms West was wrong to continue backing Sir Keir.

But Ms West doubled down on her warning, saying she would still seek the support of the 81 MPs needed to launch the challenge if the ultimatum was not met tomorrow.

Here’s a look at how a potential Labor leadership contest might play out:

There is no formal vote of confidence procedure to oust a Labor leader; This means challengers must rally the support of the party in parliament to oust them.

Any candidate would need the support of 20 per cent of the party in the House of Commons (currently 81 MPs).

Ms West has vowed to launch a fight, with the support of 10 MPs so far. This falls well short of the required nominations, but more are likely to support him to oust Sir Keir.

But his move is aimed at mobilizing a hopeful potential leadership from the Cabinet.

Written nominations will need to be submitted to Labor general secretary Hollie Ridley, which will then trigger the contest.

In the event of a successful challenge, Sir Keir will be on the ballot as the incumbent by default and will not need to field candidates. But if Sir Keir steps down, there will automatically be competition for a new leader.

It is the responsibility of Labour’s National Executive Committee to determine the timeline for the leadership election.

As talk turned to the prime minister’s political future, some drew comparisons to Labor predecessor Sir Tony Blair’s exit from 10th place; But unlike then, when Gordon Brown was the obvious candidate to replace him, Sir Keir has no obvious successor.

Speculation is rife around the ambitions of her former deputy Ms Rayner and Manchester Mayor Mr Burnham, although the former is still awaiting the outcome of a review into her tax affairs and the latter is not currently an MP.