CANBERRA (REUTERS/AFP) - Mr Kevin Rudd ousted Prime Minister Julia Gillard as head of Australia's Labor Party in a leadership ballot on Wednesday, with the country's first female leader to quit politics after national elections in September.

In toppling Ms Gillard, Mr Rudd is reclaiming the job of being prime minister on his third attempt and with elections less than three months away.

Mr 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.

Ms Gillard must now advise Governor-General Quentin Bryce that she is resigning as prime minister, which is expected to happen on Thursday, before Mr 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.

The vote of Labor Party lawmakers followed months of destabilising leadership speculation, fuelled by opinion polls pointing to an election wipeout for Labor against the Conservative Opposition headed by Tony Abbott.

Mr Rudd's dramatic return as prime minister came almost three years to the day after he was dumped in favour of Ms Gillard, and following two aborted leadership attempts, the most recent in March.

Mr Rudd was elected prime minister in November 2007 and his government helped Australia avoid recession during the 2008 global financial crisis, although he struggled to pass key reforms through parliament.