"I am putting a pin in the delusion (that an ambitious agreement is possible)," Garnaut said in an interview published in the Sydney Morning Herald newspaper on Tuesday.

"The most important thing is to get a comprehensive agreement and get it started," he said.

Rudd's target is to cut emissions by 60 percent of 2000 levels by 2050. Economic analysts warn a more ambitious reform trajectory is unlikely with ongoing panic in international financial markets.

Garnaut earlier this month said Australia should aim to cut emissions by at least 10 percent by 2020, stabilising carbon dioxide emissions at 550 parts per million in the atmosphere, or by up to 25 percent and 450 ppm if the government opted for a tougher target.

Garnaut also recommended carbon be sold for an initial A$20 (US$16) a tonne from 2010, with only marginal increases for the first two years to help business adjust.

Green groups and climate scientists have criticised Garnaut's "practical" 10 percent target as inadequate to help the world stave off potentially catastrophic climate change.

But Garnaut said it was "delusional" to have a target of 450ppm but no "practical program to allocate the burden of exactly who does what, of how to get there".

"It would set us back, not forward, to have a delusional agreement. Twelve years later, we can't afford another Kyoto," he said.

Rudd's government has promised only to consider the Garnaut review in deciding Australia's 2020 target for curbing carbon output and designing an emissions scheme to be set in law next year and begin operating in 2010. (Reporting by Rob Taylor, editing by Michael Perry)