A legal action aimed at overturning the Labour Party's decision to guarantee Jeremy Corbyn a place on the leadership ballot is due to get under way.
Labour donor Michael Foster, a former parliamentary candidate, is bringing the claim at London's High Court against the party's general secretary Iain McNicol, who is being sued in a representative capacity, and Mr Corbyn.
The case, to be heard on Tuesday by Mr Justice Foskett, follows the decision of Labour's National Executive Committee (NEC) that the incumbent leader should automatically be included in the contest.
Ballot papers will start to be sent out on August 22 with the result announced at a special conference in Liverpool on September 24.
NEC members wrestled with legal advice for six hours over whether Mr Corbyn would need to secure the backing of 20% of Labour MPs and MEPs - 51 nominations - to make it on to the ballot paper after both sides insisted the party rule book backed their case.
Mr Foster wants to reverse the NEC's decision and is seeking a declaration that, under Labour Party rules, Mr Corbyn must obtain the requisite number of nominations before his name may appear on the ballot papers.
Mr McNicol announced a leadership election on July 11 after Wallasey MP Angela Eagle obtained the necessary number of nominations.
Ms Eagle withdrew from the race a week ago in order to back Owen Smith as a "unity candidate" to take on 67-year-old Mr Corbyn, who became leader in September last year.
Mr Smith, 46, who was elected to Parliament in 2010 as MP for Pontypridd and later promoted to Shadow Secretary of State for Work and Pensions, was one of a series of shadow cabinet members to resign over Mr Corbyn's leadership.
Last week, Master Victoria McCloud backed Mr Corbyn's bid to become a party in the action, saying that he personally faced "jeopardy" in the election and his interest in the litigation was significantly different to the general body of members represented by Mr McNicol.
She said that much turned on the outcome of the election as it would determine a crucial constitutional role in the UK.
Mr Corbyn is expected to attend the hearing, which is due to last one day.