Labour MP David Cunliffe has given an assurance he will not challenge David Shearer for the leadership when it comes to a vote on 4 February.
Under constitutional changes approved by Labour's annual conference last year, there must be a vote of confidence in the leader in February, opening the way for a potential challenge.
But Mr Cunliffe, who was demoted after the conference for failing to rule out a challenge, said on Thursday Mr Shearer's position is safe.
He said he wants to be a constructive member of the Labour team and help the party win next year's election.
