Kevin Rudd supporter Kim Carr has resigned from Prime Minister Julia Gillard's outer ministry.
Senator Carr, who was human services minister, on Friday became the third minister to resign, behind cabinet ministers Martin Ferguson and Chris Bowen.
The exodus followed Mr Rudd's decision on Thursday not to challenge Prime Minister Julia Gillard after another minister, Simon Crean, called for a leadership spill.
Mr Crean was sacked by a victorious Ms Gillard on Thursday.
Senator Carr, who also backed Mr Rudd when he challenged in February 2012, said he had not made his decision lightly. 'Given yesterday's events, and the current circumstance, I have thought the principal course of action was to offer the prime minister my resignation,' Senator Carr said.
'And she has accepted it.'
The Victorian senator said he had always acted within a commitment to serve the Labor party and the federal government.
'I leave the ministry without rancour and I will continue to work for that Labor mission,' he said.
Senator Carr is half way through his upper house term.
Senator Carr said this week was the last opportunity to produce 'a Labor team which ... would have the strongest possible opportunity for Labor to remain in government'.
'I solemnly believe that Kevin provided the best opportunity and that's clearly what the evidence suggests across this country,' he said.
'We have no option but to defend the caucus decision and work for the re-election of a Labor government under the present arrangements.'
Senator Carr said he had advised Mr Rudd on Thursday not to run, when it was clear there wasn't a solid majority of caucus members supporting him for the leadership.
'While the results would have been very, very close, the worst result for Labor was for there to be a narrow loss for Kevin, because it would have been the most difficult of circumstances for all of us to be able to argue,' he said.
Senator Carr said he would like to see future leaders of the Labor party directly elected by both the caucus and grassroots members of the party.
He said Labor could win if it articulated the party's values and 'strong Labor policy'.