Labour is challenging a High Court ruling giving recent members a vote in its leadership contest, with the appeal hearing expected on Thursday.
The party lost a legal challenge to its rules banning anyone who joined as a member after 12 January from taking part unless they paid an extra £25.
Shadow chancellor John McDonnell had urged Labour not to appeal.
But the party said it would defend the right of its governing NEC "to uphold the rule book".
The court's decision, handed down on Monday, could add anywhere between 126,592 and 150,000 people to the list of those eligible to vote in the contest - according to different estimates.
http://www.bbc.com/news/uk-politics-37009871