The UN, Europe and the world's small island states reacted with disappointment and green groups voiced fury after Japan on Friday slashed its goal for greenhouse gas emissions.
UN climate chief Christiana Figueres expressed "regret" at the move and others at the worldwide negotiations in Warsaw attacked it as a fresh blow to an already-troubled process.
But Japan stood its ground and said the about-turn had been forced by the Fukushima nuclear accident in 2010.
The country announced it would aim for a 3.8-percent cut in emissions by 2020 over 2005 levels, replacing an earlier goal to cut them by 25 percent from 1990 levels.
"I want to assure you my country is still ambitious on climate change. My prime minister is committed to the climate change challenge," embattled envoy Hiroshi Minami told journalists at the annual 11-day talks on limiting carbon emissions.
The previous, more ambitious goal, had been based on the assumption that ever more of Japan's energy would come from nuclear power stations, which turned out not to be the case -- with no plants currently operational.
"Japan is a highly advanced economy... they have made impressive advances both in increasing their efficiency as well as solar energy investments and our sense is that Japan can and will continue those and will soon see that the current target is actually conservative," Figueres said at a separate briefing.
The European Union urged Japan to consider the consequences of its actions, while the Alliance of Small Island States (AOSIS) said the move was "a huge step backwards" for the global effort.
In a press statement, the European Commission said EU members "recognise" Japan's difficulties after the 2011 earthquake and tsunami, which shuttered its nuclear energy sector, but said it should nevertheless keep its promise.
"The European Union and its 28 Member States call on Japan to consider the implications of the new target for Japan's contribution to international mitigation action."
AOSIS, gathering poor island nations vulnerable to rising seas, accused Japan of retreating on promises it made at the 2009 UN climate summit in Copenhagen.
"AOSIS is extremely concerned that the announcement represents a huge step backwards... and puts our populations at great risk," it said.
The grouping pointed to Super Typhoon Haiyan, which ravaged the Philippines, as "just the latest in a series of climate-related extreme weather catastrophes" that would increase under global warming.
UN members are striving to conclude a greenhouse-gas pact by 2015 that would limit average surface warming to 2.0 degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.
The new Japanese target actually represents an increase of more than three percent on 1990 emissions levels, according to the Japanese environment ministry.
"We were expecting more from the world's third largest economic country. But instead they are racing to the bottom," complained Wael Hmaiden, head of Climate Action Network International.
'Tipping point' feared in climate talks
According to monitoring group Climate Analytics, Japan's new target will widen the global "emissions gap" -- the gulf between international carbon pledges and what is needed to meet the 2 C target -- by 3-4 percent.
"This historic reversal in Japan's climate policy away from reductions towards increasing its own emissions could become a tipping point, feeding a downward spiral of ambition globally," said policy analyst Marion Vieweg.
Japan is the latest developed country to scale back carbon commitments.
Canada walked away from the Kyoto Protocol in 2011 after years of exceeding its carbon cap, while Australia's new conservative government this week moved to scrap a carbon tax on big emitters.
According to the International Energy Agency (IEA), Japan in 2011 was the fifth biggest emitter of carbon dioxide, the principal greenhouse gas, after China, the United States, India and Russia.
The climate process suffered another blow with the announcement by Brazil overnight that deforestation of Amazonia had risen by 28 percent over the past year.