Image copyright PA More than 125,000 Labour members not entitled to vote in the party's leadership contest could now be allowed to take part after a court ruling.
Five members have won a High Court battle with Labour, which had banned anyone who joined after 12 January from voting unless they paid an extra £25.
The party is to appeal against the ruling, which is thought likely to benefit leader Jeremy Corbyn over challenger Owen Smith.
The appeal could be heard on Thursday.
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Labour shadow chancellor John McDonnell, who is running Mr Corbyn's re-election campaign, welcomed the judgement and called on Labour not to appeal it.
"We are appalled by the possibility of an unnecessary and costly appeal. If it is taken forwards, the party will be using members money to try to stop members from voting. This is unacceptable.
"I'm calling on Owen Smith to join with us in backing party members and calling on the Labour Party not to appeal and attempt to disenfranchise members."
A spokesman for Mr Smith said: "I'm sure the party will want to study the judgment and determine its next steps."
'Breach of contract'
The five members that brought the legal challenge argued the ban amounted to a breach of contract, saying they had "paid their dues" for a right to vote.
Ballot papers are due to be sent out on 22 August, with the outcome of the leadership election scheduled for 24 September.
Labour leadership election timetable
- 22 August: Ballot papers start to be sent out in the post (Labour Party members only) and by email
- 21 September: Deadline for ballot papers to be returned is midday
- 24 September: The result will be announced at a special conference in Liverpool
The High Court case was triggered by the decision by Labour's governing body, the National Executive Committee's (NEC), that full members could only vote if they had at least six months' continuous membership up to July 12 - the "freeze date".
But the party offered a window from 18-20 July when they - and non-members - could pay £25 to become "registered supporters" and gain the right to vote.
Delivering his judgement, Mr Justice Hickinbottom said: "For the party to refuse to allow the claimants to vote in the current leadership election, because they have not been members since 12 January 2016, would be unlawful as in breach of contract."
Image copyright PAHe said at the time each of the five joined the party "it was the common understanding, as reflected in the rule book, that, if they joined the party prior to the election process commencing, as new members they would be entitled to vote in any leadership contest".
The court's decision could add as many as 126,592 people to the list of those eligible to vote in the contest - Labour's membership stood at 388,407 on 10 January and the current membership figure is 515,000.
It is not known how many of these new members paid the £25 to get a vote in the contest or whether those that did will now have to be reimbursed.
There is also no way of verifying the motivations of those who have signed up in the past six months but both leadership teams say overturning the ban is likely to be beneficial to Mr Corbyn.
'Equal and valued'
Reacting to the ruling Edward Leir - one of the five who brought the legal challenge - said the court's ruling was a "victory for equality and inclusion".
"This judgement is a vindication that the political process should be fair, democratic and inclusive; that political parties, like any other organisation, must uphold its rules fairly to those who support them," he added.
The others in the group were Christine Evangelou, Hannah Fordham, Chris Granger and "FM", a new member aged under 18.
They have been crowdfunded and are seeking to raise £40,000 to cover their legal costs.
Kate Harrison, solicitor for the claimants, said it was "a good day for democracy, a good day for my clients who are proud to be members of the party that stands for social justice, and a good day for the Labour Party".
"This case was about the right to vote under the Labour Party constitution, under which all members are equal and valued," she said.
She told BBC News claims the ruling would benefit Mr Corbyn were based on "speculation" and the case had "nothing to do with who the different claimants would vote for".
She said the £25 charge had "excluded people who could not afford £25" and younger members who were "excited about their right to vote".
BBC political correspondent Iain Watson said Labour would have to "go back to the drawing board" and find a way of "re-enfranchising" these members if it loses a subsequent appeal.