The five Labour leadership candidates will face each other at a hustings later in a bid to gain support from one of the UK’s biggest trade unions.
GMB, which represents workers and has 620,000 members, is due to announce who it is backing after the event.
So far only Sir Keir Starmer has gained the minimum threshold needed to go on to the final ballot.
Rebecca Long-Bailey, Emily Thornberry, Lisa Nandy and Jess Phillips are yet to to receive the necessary backing.
The candidates have to secure nominations from 5% of constituency Labour parties (CLPs) or at least three affiliate groups – at least two of which are trade unions – comprising of 5% of affiliated members.
Tuesday’s hustings will take place in front of GMB’s executive.
Sir Keir will visit the Union of Shop, Distributive and Allied Workers (Usdaw) in Manchester later, after it announced its support for him on Monday.
The shadow Brexit secretary also has the backing of Unison and affiliate group the Socialist Environment and Resources Association (SERA).
Usdaw’s general secretary, Paddy Lillis, said: “The Labour Party must be led by someone who can persuade voters that they have what it takes to be a prime minister and we are a government-in-waiting.”
Sir Keir said he was “honoured” to have the backing of Usdaw, adding: “If I’m elected leader, Labour will stand shoulder to shoulder with the trade union movement as we take on the Tories and rebuild trust with working people.”
Meanwhile, shadow business secretary Mrs Long-Bailey is due to speak at a campaign event in London.
Usdaw has also given its support to shadow education secretary Angela Rayner for deputy leader.
She faces competition for the deputy leadership from Scotland’s only remaining Labour MP, Ian Murray, shadow equalities minister Dawn Butler, Tooting MP Rosena Allin-Khan and shadow justice secretary Richard Burgon.
One of these five candidates will replace Tom Watson, who stepped down as deputy leader in December.
The unions, affiliate groups and CLPs have until 14 February to get their preferences submitted.
The final ballot of party members, trade unionists and registered supporters will open on 21 February, and the new leader and deputy will be announced on 4 April.