Millbank, which has been accused of "control freak" tendencies, is being sued after excluding a pro-Livingstone union from the ballot to select the party's candidate.
Members of the Manufacturing Science Finance union said Labour acted unlawfully when it declared 15,000 votes null and void in an alleged bid to boost the chances of Frank Dobson, who is favoured by Tony Blair.
The six MSF activists taking the action, to be heard on 13 January, said a judgment in their favour could force the party to reconsider bans on the votes of other party affiliates. London members of the RMT transport union, the train drivers' Aslef union and the Bectu broadcasting union have been refused a vote.
Labour's high command said the London area of MSF failed to pay its party affiliation fees by the 31 December 1998 deadline and therefore forfeited its right to participate in the selection process.
Lawyers for the "MSF six" said Labour did not warn the union that its voting power would be removed when it paid the pounds 1 a head subscription in July this year and could not lawfully introduce a ban subsequently. The litigants, members of the union's London council, said the region regularly paid subscriptions late and had never been disenfranchised until now.
The left-wing London executive of the union overwhelmingly backed Mr Livingstone and still hopes to recommend him in a ballot. The union's national leadership has suspended two of the six activists for defying its instructions and allegedly authorising litigation in the name of MSF.
Roger Lyons, general secretary, said the MSF's official legal advice was that Labour had acted lawfully. The activists subsequently decided to fund litigation themselves.
John Barr, a member of the London council of the union, said the members involved regretted taking Labour to court but he believed they would win. "It is even more regrettable that our own general secretary has seen fit to side with the Millbank control freaks," he said.
Mr Lyons, who is understood privately to oppose Mr Livingstone, denied acting on behalf of party officials when he tried to block legal action.
He has supplied a statement to the court in which the union's national executive urges the Labour Party leadership to reconsider its decision.
The MSF leader said the six activists had been invited to submit their own legal advice to the executive but had failed to do so. He pointed out that 3,000 members of the eastern division of MSF would be voting in the poll and there had been no attempt to disenfranchise them, because fees were paid on time.
A Labour spokesman said party rules dictated that affiliates that did not pay promptly forfeited the right to vote.
He described as "complete garbage" any suggestion that the party was trying to undermine Mr Livingstone and pointed out that other unions were balloting their members.Reuse content