The EPMU executive will this morning endorse Andrew Little in the four-way Labour leadership contest.
Mr Little is a former national secretary of the Engineering Printing and Manufacturing Union.
The EPMU executive decided unanimously to support Mr Little against Grant Robertson, David Parker, and Nanaia Mahuta.
Former president Mike Williams backed Mr Parker at the weekend and former Deputy Prime Minister Michael Cullen last week backed Mr Robertson.
Under Labour's rules, unions may recommend which candidates delegates and members may vote for but cannot insist.