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 Mr Corbyn needs the nomination of 51 MPs and MEPs to stand in the contest.
The NEC has been presented with conflicting legal advice over Mr 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 Mr 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, almost two thirds (63%) of respondents said he was doing badly as leader compared with a third (33%) who said he was doing well.
More than three quarters (76%) said it is unlikely that Mr 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."
Meanwhile, deputy leader Tom Watson told a meeting of the parliamentary Labour Party (PLP) that his abandoned peace talks with union leaders failed to find a way around the impasse between MPs and pro-Corbyn elements of the party.
He said: "For years I've been told I'm a fixer.
"Well I've tried to fix this, I've really, really tried, and I've failed.
"I've tried to find a way forward for the party between two apparently irreconcilable decisions.
"Clearly the vast majority of the PLP has already made it clear they wouldn't countenance a settlement that involved Jeremy staying in place."
Labour's civil war intensified after the EU referendum when 172 of the party's MPs indicated that they had no confidence in Mr Corbyn in a vote in which he garnered the support of just 40 Westminster colleagues.
Mr Watson's spokesman said the deputy leader acknowledged that Mr Corbyn had a big mandate from members, just like he did in his elected role.
But Mr Watson told the PLP "with power comes responsibility and if 80% of the PLP had said they had no confidence in me I would resign, despite the fact I have a big mandate from members".