Tens of thousands of new members of the Labour Party are set to find out today whether they have won the latest round of a legal battle for the right to vote in the forthcoming leadership election.
Court of Appeal judges are being asked to overturn a High Court test case ruling on Monday in favour of five members who say they were unlawfully "frozen out" of the high-profile contest between Jeremy Corbyn and rival Owen Smith.
Iain McNicol, the party's general secretary, is leading the appeal against Mr Justice Hickinbottom's decision that swept away a ban on recruits who joined after January 12 getting the vote.
If the appeal is rejected, Mr Corbyn could receive a significant boost in his fight to remain party leader, as most new members are expected to support him.
Labour's National Executive Committee (NEC) decided on July 12 - referred to as the "freeze date" - that full members would not be able to vote if they had not enjoyed continuous party membership for at least six months.
But the High Court declared that refusing them the vote would be an unlawful breach of the party's contract with members as set out in the rule book.
Lawyers for the five - Christine Evangelou, the Rev Edward Leir, Hannah Fordham, Chris Granger and "FM", a teenage member - had argued that the NEC had no power under the rules to retrospectively freeze a full member's ability to vote in leadership elections.
They say approximately 150,000 individuals joined the party between January 12 and July 12 and their ability to vote is at stake.
Clive Sheldon QC, appearing for Mr McNicol and the Labour Party, asked three appeal judges on Thursday to set aside the High Court decision.
Mr Sheldon submitted that the rule book gave the NEC "sufficiently broad powers that it can actually override the rules framework in a particular case, if it so wishes."
He described the NEC as "the guardian of the constitution" and said nowhere did the rule book say a freeze date could not be retrospective.
David Goldstone QC, appearing for the five members, told the judges the appeal should be rejected as the NEC lacked authority under the rules to act retrospectively.
He submitted:"We say there are obvious dangers in powers that allow for retrospective exclusion of existing members from the franchise... There is scope for abuse".
Mr Goldstone said the rules should be construed in a way so that was not possible.
After a day-long hearing, Lord Justice Beatson, sitting with Lady Justice Macur and Lord Justice Sales, said the court appreciated the urgency of the case and aimed to give its ruling at 3pm on Friday.
There has been speculation that Mr McNicol could face being ousted if the party loses its appeal as divisions within the party deepen still further.
A senior Labour source said: "If Labour loses the appeal, the position of Iain McNicol becomes untenable."