Transport Minister Anthony Albanese will vote for Kevin Rudd in Monday's ballot for the prime minister.
Announcing his decision at a press conference this morning an emotional Mr Albanese said he had offered his resignation as Leader of the House to Prime Minister Julia Gillard.
"I have despaired in recent days as I have watched Labor being devalued," Mr Albanese said.
He spoke about Labor's "good record" in government and the need for the party to take on their conservative opponents.
The minister called for unity behind whoever might be prime minister after Monday's ballot.
Mr Albanese is the last remaining senior minister to declare his hand. Most have said they would vote for Ms Gillard.
Later, speaking at the Country Labor conference in Cessnock, Ms Gillard said she had refused to accept Mr Albanese's resignation as he was doing a "remarkable job" as leader of government business in the house.
She said while she disagreed with Mr Albanese's decision to support Mr Rudd, she knew he would continue to work hard for the party and for Labor unity, regardless of the result of Monday's vote.