General Secretary Len McCluskey is thought to favour Rebecca Long-Bailey in the contest, who’s views are closely aligned with current leader, Jeremy Corbyn.
Labour are looking to recover from their worst election result in a generation and to rebuild the red wall of seats in the North.
Speaking to Sky News, McCluskey, said: “What Jeremy Corbyn’s done with Labour, and that’s his legacy, is to create this alternative, and I think that’s the right thing to do.”
Three other hopefuls remain alongside Long-Bailey in the contest. Kier Starmer is the bookies’ favourite to take the job, while Lisa Nandy and Emily Thornberry remain outsiders.
Thornberry is facing criticism over her choice to send her son to a partially-selective school. The MP for Islington South said on Wednesday she would “never apologise for doing the best for my kids”.