Chief Cabinet Secretary Yoshihide Suga said the new target for 2020 -- 3.8 per cent below 2005 levels -- replaces an ambitious goal to slash emissions by one-quarter from 1990 levels, which had been hailed by environmentalists.
Japanese media said the new target -- which accounts for idling the country's nuclear reactors after the worst atomic accident in a generation -- represents a 3.0 per cent rise over levels in 1990, the base year for the Kyoto Protocol.
"Our government has been saying... That the 25 per cent reduction target was totally unfounded and wasn't feasible," he told reporters in Tokyo.
Hatoyama had said the nation would slash its carbon emissions provided other major polluters such as China and the United States also made sharp reductions.
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A foreign ministry official today said the new target would likely be announced next week at a 12-day climate talks summit in Warsaw which kicked off on Monday.
He added that the target was "temporary" given Tokyo's desire to switch on idled nuclear reactors, an idea that has been met with strong public resistance.
The earthquake and tsunami of March 2011 sent reactors at the Fukushima nuclear plant into meltdown and generated widespread distrust of a technology previously relied on to provide around a third of Japan's electricity.
Prime Minister Shinzo Abe's business-friendly Liberal Democratic Party ousted the Democratic Party in December elections after pledging to review the emissions cut target in light of the post-Fukushima switch to fossil fuels.
In statement issued before today's announcement, conservation group WWF condemned a pullback on Japan's commitments, saying it could dent progress at the UN climate talks in Poland.
"This move by Japan could have a devastating impact on the tone of discussion... In Warsaw," it said.
"It could further accelerate the race to the bottom among other developed countries when the world needs decisive and immediate actions to 'raise' ambition, not to 'lower' ambition.