Jeremy Corbyn is ready to challenge Labour over a so-called "purge" in which voters in the leadership election are being barred from casting a ballot supposedly because they support another party.
Mr Corbyn has said that if significant numbers are banned "unfairly" then the decision to reject them "must be looked at again and challenged".
Many voters who have recently joined Labour as registered supporters or as affiliate members are being told their votes will not count as the party works to root out potential infiltrators into the leadership race.
The contest has been marred by concerns over "entryism" from hard left individuals or Tory supporters who could seek to turn the outcome by signing up as registered supporters for £3 under new party rules.
Some fear that the rules, introduced by Ed Miliband, could see surprise frontrunner Mr Corbyn win the contest due to support from voters who have previously opposed Labour.
But many of those who appear to have been barred are complaining on social media that they voted Labour in the last election but are still having their votes blocked.
Speaking after a rally in Nottingham, Mr Corbyn revealed he was talking to Labour officials about the issue.
He said: "I know there are a number of people that it seems slightly odd have been not allowed but there is a process of looking again at many of these applications. I'm obviously in touch with the Labour party officials on this.
"The staff of the Labour party both in London and Newcastle have worked incredibly hard to try and deal with the massive number of applications.
"I hope these little glitches can be ironed out. But the vast majority of people that registered online or through text messaging have been accepted, are getting ballot papers and are getting a voting opportunity.
"Yes, if there are significant numbers rejected in apparently an unfair way, then of course that must be looked at again and challenged. But the issue really is let's be happy about getting 600,000 people taking part in an election."