Jeremy Corbyn is preparing to deliver a speech to Britain's biggest trade union with Labour's ruling committee poised to decide whether he should automatically be allowed to take part in the upcoming leadership race.
The Labour leader will speak at the Unite policy conference in Brighton as the party's National Executive Committee (NEC) meets in London before ruling on whether Corbyn needs the nomination of 51 MPs and MEPs to stand in the contest.
The NEC has been presented with conflicting legal advice over Corbyn's position, with Labour-commissioned analysis stating that he will need the nominations, but Unite-backed advice from Michael Mansfield QC concluding that he does not because he is a sitting leader.
The battle looks more likely to end up in the courts with Corbyn vowing to fight any exclusion from the ballot paper, but the anti-Corbyn camp weighing up a legal challenge if he is allowed to stand without the nominations.
Meanwhile, a YouGov poll for the Election Data website found the Labour leader's union backing dissolving.
In the survey of 1,221 trade union members from Unite, the GMB, Unison, USDAW and the CWU, more than three quarters (76%) said it is unlikely that Corbyn will ever become prime minister and more than two thirds (69%) said it was unlikely Labour would win the next election with him as leader.
The poll is significant as 12 of the NEC seats - around a third - are taken up by union representatives.
Angela Eagle, who has met the nominations threshold, launched her leadership challenge on Monday, promising to make Labour electable again after the "howl of pain" expressed in the Brexit vote by people who felt they had been ignored for too long.
And she insisted it was time for Labour to have a woman leader, amid reports that former frontbencher Owen Smith could launch a rival leadership challenge.
Ms Eagle told Channel 4 News: "The Conservatives have their second woman prime minister.
"The Labour Party, the party of equality who pioneered anti-discrimination - it's about time they had their first elected woman leader."