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.
THe EPMU is the largest of six unions affiliated to the Labour Party and, combined, the unions have 20 per cent of the vote. The caucus has 40 per cent and the membership 40 per cent.
A round of 15 candidates meetings for members begins in Wellington on Wednesday and the result will be announced on November 18.
Former leader David Cunliffe resigned after the September general election.