Labour is to use canvassing details gathered during the general election to weed out infiltrators trying to skew the leadership contest.
The party's procedure committee has decided the information should be deployed after the idea was raised at a crunch meeting with the four contenders yesterday.
A full breakdown of candidates' backing from union members, party members and registered supporters will also be published with the final result on September 12 in a bid to assuage concerns about the integrity of the process.
The move comes amid fears that Tories and the hard-left are attempting to secure victory for surprise frontrunner Jeremy Corbyn.
A Labour Party spokesman said: "The procedure committee met today to receive feedback from yesterday's candidates' briefings. The following decisions were made:
"Canvassing material will be used to inform work to establish whether participants are supporters of the aims and values of the Labour Party.
"That as well as the one-person, one-vote overall result, the results of the Leader and Deputy Leader Elections will be broken down by Party Members, Affiliated Supporters and Registered Supporters. No other breakdown of the results will be provided."