Jeremy Corbyn's campaign team has blasted a Court of Appeal ruling that will stop new party members from voting in Labour's upcoming leadership election.
Labour's ruling body won its bid to overturn a High Court decision that had paved the way for around 130,000 supporters who signed up from January to take part in the contest.
Many of the members affected are believed to back Corbyn rather than his rival Owen Smith.
A spokesman for Corbyn's campaign team said: "We think that this is the wrong decision - both legally and democratically.
"The Court's ruling disenfranchises nearly 130,000 Labour members, who joined the party since January and were explicitly told that they would have a vote in any leadership election."
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.
The High Court on Monday ruled in favour of five members who said they were unlawfully "frozen out" of the leadership battle.
Iain McNicol, the party's general secretary, led the appeal against Justice Hickinbottom's decision.
Announcing the Court of Appeal's decision on Friday, Lord Justice Beatson, sitting with Lady Justice Macur and Lord Justice Sales, said: "On the correct interpretation of the party rules, the National Executive Committee has the power to set the criteria for members to be eligible to vote in the leadership election in the way that it did."
An application by the five for permission to appeal to the Supreme Court was rejected - but it is still open for them to apply directly to the highest court in the UK if they want to attempt to take their case further.
Corbyn's team said Labour's lawyers had "invoked an obscure clause" in the party's rule book that "could be read as giving the NEC the right to ignore all of the rules laid out for leadership elections".
A spokesman said: "In other words, this is a 'make it up as you go along' rule. We do not think that making it up as you go along is a reasonable way to conduct democracy in our party.
"Serious questions must be raised, however, over why and how the NEC Procedures Committee brought this appeal. In doing so, it effectively risked new members' money on an attempt to disenfranchise them. If we are to build a big, inclusive party to take on the Tories, we need to secure democracy in our party."
NEC chairman Paddy Lillis insisted the party had been right to launch an appeal. He said: "The Labour Party welcomes the decision of the appeal court. The party has said consistently throughout this process that we would defend vigorously the decisions of the NEC."
Smith, who is expected to benefit from the ruling, said: "I had welcomed the prospect of 125,000 additional members being given the opportunity to vote in this vitally important leadership election.
"The decision of the Appeal Court today doesn't change my approach to this contest; I am getting on with the job of talking to as many members and supporters across the country as possible and making the case for a united, radical and credible Labour Party."