Kevin Rudd says he didn't run for the leadership because he was told there was "zero" chance of winning a caucus ballot.
Prime Minister Julia Gillard called a spill of the Labor leadership on Thursday, but Mr Rudd did not challenge despite caucus members urging him to do so.
Mr Rudd told reporters in Brisbane on Friday that had he challenged he would have been attacked for going back on a promise made in February 2012 never to challenge again and "split the Labor Party right down the middle".
He said he was advised in a meeting with supporters on Thursday that he did not have a majority of votes.
"I asked them `what are the prospects of us obtaining a significant majority' - their collective response was zero," he said.
"I asked them `what are the prospects of us obtaining a majority' - to which their response was zero." Then, according to Mr Rudd, each told him "Kevin I believe you should not run".
Mr Rudd said was time for the Labor party to "unite totally" behind Ms Gillard. "... Julia Gillard, has my 100 per cent support," he said.
"As I said in a written statement today, there are no circumstances under which I would return to the leadership of the Australian Labor Party in the future."
He would, he said, continue to offer his services as the federal member for Griffith.
Mr Rudd said it was time to confront a significant threat to Australia's future. "That is Tony Abbott and all that he stands for.
A person I have said before I don't believe has the temperament or the polices to be the prime minister of the country."
He said the party must present to Australia its "phenomenal achievement" in keeping Australia out of the world recession that followed the global financial crisis.
