CANBERRA (Reuters) - Australian Greens leader Bob Brown, whose party supports Prime Minister Julia Gillard's minority government, resigned as his party's leader on Friday and announced he would leave parliament in June.
The resignation stunned Greens lawmakers, but will not affect their agreement to support the government.
As a Senator, Brown's resignation also has no impact on the numbers in parliament and will not force a by-election, as his seat will be filled by a Greens replacement who will fill the casual vacancy.
Brown, 67, has been a senator for the island state of Tasmania for 16 years and is the most powerful political figure in Australia outside the ruling Labor party and the opposition Liberal and National parties.
"It is prime time to hand over the reins," Brown said.
He will be replaced as leader by Greens deputy leader Christine Milne.
Brown said his resignation would not affect the Greens agreement to support Gillard's government, which has a one seat majority with the support of the Greens and two independent lawmakers.
"The arrangement with the government stays the same," Brown said, adding he had informed Gillard about his resignation.
Brown has been a towering figure in Australian politics since he first attracted national prominent by protesting against a major dam project in Tasmania in the early 1980s.
He has helped build the Greens into a significant force in Australia, with the party winning a record high vote of around 12 percent at the 2010 elections.
Under his leadership, the Greens went from one seat in the upper house Senate a decade ago to now have nine seats and the balance of power in the Senate, and the party has one seat in the lower house of Representatives.
Milne said she would work to build on Brown's legacy.
"For 25 years, Bob has been an inspiration to millions of Australians and a great force for good in our country," Milne said.
(Reporting by James Grubel; Editing by Paul Tait)