Judges ruled in favour of five supporters who accused the party’s National Executive Committee (NEC) of unlawfully “freezing” them out, even though they had “paid their dues”, when it decided that full members would not be able to vote unless they had six months’ continuous membership up to 12 July.
Party officials said Labour would mount a legal challenge to “defend the NEC’s right” to uphold its rules.
A Labour spokesman said: “The procedures committee of the NEC has decided that the Labour Party will appeal this ruling in order to defend the NEC’s right, as Labour’s governing body, to uphold the rule book, including the use of freeze dates.”
The leadership race was plunged into chaos by the court ruling and members of Labour’s procedures committee discussed how to respond in a hastily arranged conference call.
Labour was given permission to go to the Court of Appeal but Jeremy Corbyn’s allies had urged the party not to pursue the option, claiming members’ money was being squandered on stopping them from voting.
General secretary Iain McNicol faces being ousted if the party loses its legal bid.
A senior Labour source said: “If Labour loses the appeal, the position of Iain McNicol becomes untenable.”
Conservative Party chairman Sir Patrick McLoughlin said: “This summer, the Labour Party have held more High Court battles than leadership hustings.
“At a time when the people of Britain look to a party who will fight for them and their family, Labour are spending their summer fighting amongst themselves.”
In July, a judge rejected a challenge to Labour’s decision to automatically put Mr Corbyn on the ballot paper.
Mr Corbyn’s challenger Owen Smith yesterday branded the Labour leader “useless” and said he would be tempted to hold a snap election if he was the Conservative prime minister.
Mr Smith also underlined his demand for a second referendum to allow voters to sign off any Brexit deal.
He said he entered the shadow cabinet “with a bit of hope” but lost faith when Mr Corbyn failed to act on his promises.