The High Court is due to hear a legal challenge to the Labour Party's decision to guarantee Jeremy Corbyn a place on the leadership ballot.
Labour donor Michael Foster, a former parliamentary candidate, is bringing the legal action against the party's general secretary Iain McNicol, who is being sued in a representative capacity.
Mr Corbyn requested to be added to the proceedings as second defendant.
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.
NEC members wrestled with legal advice for six hours over whether Mr Corbyn would need to secure the support of 20% of Labour MPs and MEPs (51 nominations) to make it on to the ballot paper, after both sides maintained that 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 secure the requisite number of nominations before his name can 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 has faced intense calls to step down.
Mr Corbyn is expected to attend the hearing, which is due to last one day.