Corbynsceptic MP Jess Phillips has announced in a video message to her supporters that she is pulling out of Labour's leadership election, leaving four candidates still in the running.
Affiliated societies - such as the Jewish Labour Movement, the Fabian Society or Labour Business - will lend their backing to candidates, but those running will also need the support of two affiliated trade unions - like the GMB, Aslef or Unite.
She had yet to receive any nominations from trade unions, affiliate bodies or local parties.
Sir Keir is the frontrunner in the Labour contest, having already secured his place on the final ballot paper as a result of nominations from the unions Unison and Usdaw and the Socialist Environment & Resources Association (Sera), an affiliate group.
Now in last place after Ms Phillips abandoned her leadership bid, shadow foreign secretary Emily Thornberry claimed that if she were elected Labour leader, she would "frighten the life" out of Boris Johnson during Prime Minister's Questions.
Responding to the announcement Ms Nandy said: "Labour is at a crossroads".
Ms Phillips said the event was "awful" as she also revealed she believes she is unlikely to win the contest.
The GMB also also nominated Angela Rayner to be Labour's deputy leader, with Mr Roache saying: "With Lisa and Angela at the helm, Labour can do things differently".
Five MPs passed this threshold: Keir Starmer (68 nominations), Rebecca Long-Bailey (26), Lisa Nandy (26), Jess Phillips (22) and Emily Thornberry (10).
The Labour Party has developed a widespread base of activists who have been strong supporters of the Corbyn project since 2015 and candidates will need to appeal to it in order to be successful.
Ms Nandy - who already has the support of the National Union of Mineworkers - said she could "not be more proud", and the next leader's challenge was to "recover our ambition and inspire a movement".
Ms Nandy is a step closer to joining him after the 60 members of the GMB's Central Executive Council made a decision to back her, saying they were impressed by her "ambition, optimism and decisive leadership".
"Although she can't build the broad coalition needed to rebuild the Labour Party now, she's recruited thousands upon thousands of new members, amplified the voices of the voters and will play a big role in Labour's future".
Mr Corbyn's successor will be announced on 4 April.