Sir Keir Starmer has become the first Labour leadership candidate to secure enough nominations to enter the race.
The shadow Brexit secretary has the backing of 23 MPs and MEPs to replace Jeremy Corbyn – passing the minimum of 22 nominations.
Shadow business secretary Rebecca Long Bailey is his closest competitor, securing seven nominations so far, followed by Jess Phillips with six.
Nominations close on 13 January and the winner will be announced on 4 April.
Those running to become deputy leader of the party are holding a hustings in Parliament.
The candidates are shadow justice secretary Richard Burgon, shadow education secretary Angela Rayner, shadow women and equalities minister Dawn Butler, Scotland’s only remaining Labour MP Ian Murray, Rosena Allin-Khan and Khalid Mahmood.
Colleagues backing Sir Keir include two newly announced members of Labour’s shadow cabinet – shadow culture secretary Tracy Brabin and shadow Brexit minister Thangam Debbonaire.
Mrs Long Bailey’s backers include deputy candidate Ms Rayner, while Birmingham Yardley MP Jess Phillips has the support Wes Streeting – an outspoken critic of Mr Corbyn over his handling of anti-Semitism allegations in the party.
Wigan MP Lisa Nandy has two nominations and shadow foreign secretary Emily Thornberry has one.
However, shadow Treasury minister Clive Lewis has yet to receive the official backing of any of his colleagues.
Mr Corbyn said on Tuesday that he would not endorse any of the candidates and “won’t be saying who I’ll be voting for”.
Earlier, Mr Lewis rated Mr Corbyn “six out of 10” as leader of the Labour Party.
Speaking to the BBC’s Victoria Derbyshire programme, he criticised Mr Corbyn for making mistakes on Brexit and dealing with anti-Semitism.
“But some things he got right,” Mr Lewis added, “so in many ways he’s renewed our party.”
The comments follow those of Mrs Long Bailey, who rated Mr Corbyn 10 out of 10 for his performance as leader.