Kevin Rudd ousted Prime Minister Julia Gillard as head of Australia's Labor Party in a leadership ballot Wednesday, with the country's first female leader to quit politics after national elections due in September.
The embattled Gillard called the vote after a day of intense party-room moves to depose her in favour of her arch-rival and predecessor, who she ruthlessly deposed in 2010.
Rudd won the vote of the Labor caucus 57 to 45, the official returning officer announced, amid mounting unease in the party over an expected rout by the Tony Abbott-led conservative opposition at national elections scheduled for September 14.
Gillard must now advise Governor-General Quentin Bryce that she is resigning as prime minister, which is expected to happen on Thursday, before Rudd can be sworn into office.
Reports said Treasurer Wayne Swan, a key Gillard supporter and her deputy, had also resigned, along with a number of other frontbenchers including Trade Minister Craig Emerson.
"The mood was quite sombre," said returning officer Chris Hayes.
It was the third time since the 2010 election that Gillard's hold on power was tested.
Rudd himself launched an unsuccessful challenge in early 2012, but was routed 71 votes to 31.
Then in March this year, Labor elder statesman Simon Crean made a failed attempt to reinstall Rudd who refused to stand and said at the time he would not challenge Gillard "under any circumstances".
In the aftermath, several ministers who backed Rudd resigned while Crean was sacked.