More than 250,000 ballot papers will be sent out next month before meetings around the country where the Tory faithful will hear David Cameron and David Davis make their pitch for the leadership.
But the party's labyrin-thine rules mean some members will not be able to vote. For example, couples on reduced joint subscriptions will get one ballot paper between them.
Members who pay less than £15 a year in subscription and have joined since 1998 will also not be allowed to vote.
Critics said the rules could mean up to 50,000 paid-up members being deprived of a ballot paper.
Christopher Montgomery, a party activist, told BBC Radio 4's Today programme: "Legal action would definitely be taken if it's closer to two, rather than three, hundred thousand people who get ballot papers sent out."
A Tory spokesman said there would be no case for a legal challenge to the contest, which will be overseen, as before, by the Electoral Reform Society.
Mr Davis said: "Most notions of suffrage involve putting suffrage as wide as possible, not based on just how well off you are."