Labour lawmakers are trying to unseat leader Jeremy Corbyn, a 67-year-old left-winger considered unelectable by the party's more centrist members.
Labour legislator Owen Smith is running against Corbyn in an election that will be decided by half a million party members.
A party donor went to court, arguing that Corbyn's name should not be on the ballot because he did not get backing from 51 Labour members of Parliament a threshold Smith had to meet.