Words by ITV News Westminster Producer Lewis Denison
After a crushing defeat in the 2019 General Election, Jeremy Corbyn announced he would stand down as leader of the Labour Party - meaning for the next few months candidates will be making their case to replace him.
The race officially began on Tuesday, January 7, and the winner will be announced on Saturday, April 4, it was agreed by the party's ruling National Executive Committee (NEC).
But what happens in between those dates and how will the next leader be chosen?
Step 1: Nominations from colleagues
For a Labour leadership hopeful to become a candidate on the ballot paper they must first be an MP for the party and be nominated by 10% of its MPs and MEPs.
In the 2020 leadership contest, hopefuls will have from January 7 till January 13 to get the required 22 signatures.
If a leadership hopeful is unable to win backing from 10% of members then their application to be on the ballot will be rejected.
Step 2: Gaining backing from elsewhere
Once a leadership hopeful has gained support from enough colleagues, they must also gain support from local Labour parties - known as Consituency Labour Parties (CLPs) - or from affiliates.
Thanks to a rule introduced in 2018, candidates must gain backing from 5% of CLPs - the threshold would be around 33 CLPs - or from three of its affiliates.
Affiliates include 12 trade unions tied to the Labour Party, and associated groups, such as the Jewish Labour Movement.
Controversially, however, Labour Party rules state two of three affiliates must be trade unions, giving them enormous clout in the selection process.
It means it would not be possible for a candidate to pass this stage without either the support of local Labour parties or trade unions.
From January 15 until February 14 CLPs, unions and affiliates will be able to nominate who they want to be the next leader.
Step 3: Gaining Labour voters' support
Members of the Labour Party, affiliated supporters and registered supporters are able to vote in the leadership contest.
As of July 2019 the party had 485,000 members, according to Parliament.uk.
Affiliated supporters are people who are members of Labour's socialist societies and 12 trade unions - around 99,000 people from this category voted in 2016.
A registered supporter is someone who pays a fee to be able to vote in the election.
In 2015, when Mr Corbyn was first elected, the NEC set the amount at £3 and more than 100,000 registered.
When Mr Corbyn's leadership was challenged a year later, the figure was raised to £25 and the registration was only open for 48 hours, but a huge 180,000 still registered to vote.
The fee for registered supporters remains at £25 for the 2020 leadership election, and again they will be given 48 hours to register.
Registered supporters will be allowed to sign up to join the election process from January 14 - 16.
On January 20 there will be a freeze on new members' voting rights, meaning anyone opting for full membership after this date won't be able to vote in the election.
The postal ballot will then run from February 21 to April 2.
Final stage: Selecting the winner
The party uses the alternative vote system of preferential voting.
That means in the postal ballot voters rank candidates in order of preference and if more than half of voters have the same favoured candidate, they are elected.
If there is no overall winner, the candidate who came last is removed and votes are redistributed to voters’ next preferred candidate, repeating the process until a winner is produced.
The winner of the 2020 leadership contest will be announced on Saturday, April 4.
The system allows for one vote per person, and an NEC member told ITV News Political Correspondent Paul Brand it was agreed that there will be "extra checks to make sure people don't register their dogs this time".
Who are the likely contenders and who's no longer in the race?
Sir Keir Starmer is favourite in the race to replace Mr Corbyn.
He's up against shadow business secretary Rebecca Long-Bailey and Lisa Nandy.
Ms Long-Bailey is the likely favourite of Corbyn supporters, due to her links to and support of the outgoing leader.
Shadow foreign secretary Emily Thornberry, Jess Phillips and Clive Lewis have all dropped out of the race after failing to secure enough support.