The six affiliated unions casting votes in Labour's first party-wide leadership vote will not be allowed to bloc vote and nor will the individual delegates casting the votes.
It is a rule designed to protect the unions from accusations of running the party.
The unions will be allowed to consult their wider membership or run internal ballots within their own unions to help the voting delegates.
But they can't direct voting delegates, said Labour affiliates council vice-president Angus McConnell of the Dairy Workers' Union.
"We cannot bind people and we cannot see how they voted."