The move isolates the United States, which will now be the only developed nation not to ratify the agreement which sets binding limits on developed countries to curb the carbon emissions blamed for global warming.

"This is the first official act of the new Australian government, demonstrating my government's commitment to tackling climate change," Rudd said in a statement.

Climate scientists said the development was a major step for Australia and sent a clear message to Washington.

"This has given America no excuse now. They are now the only country who won't ratify Kyoto, they are the ones most responsible for the problem and they are not living up to their responsibility," said Barry Brook, professor of climate studies at Adelaide University.

Rudd, 50, led the centre-left Labor party to victory at the Nov. 24 election, ending nearly 12 years of conservative rule, by promising a new generation of leadership and committing to sign the Kyoto pact.

The former conservative government refused to ratify Kyoto, saying it would unfairly hurt the Australian economy with its heavy reliance on coal for energy and export income, while countries like India and China were not bound by targets.

But a new report from the environment think tank the Climate Institute, written by government and university scientists, found that Australia's economy could easily cope with strong cuts in greenhouse emissions.

It said growth would fall by only 0.1 percent of gross domestic product annually if Australia set a target of 20 percent cuts in emissions by 2020 and aimed to be carbon neutral by 2050.

"Leading the way on climate is an affordable, prudent and achievable investment," Climate Institute chief executive John Connor said on Monday.

Shortly after Rudd was sworn in, the Kyoto decision was approved by Governor-General Michael Jeffery, who represents Britain's Queen Elizabeth in Australia's constitution and who must approve all international treaties.

Under U.N. guidelines, full ratification takes place 90 days after the United Nations receives the formal Instrument of Ratification, meaning Australia will be a full member of the Kyoto club by the end of March.

The way is now clear for Rudd to play a stronger role at the U.N. climate talks in Bali, which opened negotiations on Monday on new carbon emission targets for beyond 2012. He is to lead a delegation of four Australian ministers at the conference.

The previous government said Australia would meet its Kyoto targets, despite not ratifying the pact, but Rudd said the latest advice suggested it would miss its target to curb greenhouse emissions growth to 108 percent of 1990 levels by 2012.

"We are currently likely to exceed, or overshoot, our target by one percent," Rudd said, adding that Australia faced penalties under new targets beyond 2012.

Rudd has set a long-term target of cutting carbon emissions by 60 percent of 2000 levels by 2050, but has yet to announce an interim target for emissions by 2020.