Climate change will be the focus of international talks in Copenhagen
Japan's next leader has promised a big cut in greenhouse gas emissions, saying he will aim for a 25% reduction by 2020 compared with 1990 levels.
Democratic Party leader Yukio Hatoyama is due to take over as prime minister on 16 September, after a resounding election victory in August.
His predecessor, Taro Aso, had pledged cuts of only 8%.
Mr Hatoyama said the plan was dependent on other nations agreeing targets at December's climate talks in Copenhagen.
Richard Black, BBC environment correspondent
Mr Hatoyama's target puts Japan alongside the EU in pledging substantial greenhouse gas emissions by 2020.
Japan's plan is conditional on achieving a deal at the UN summit in December, so it presents an additional "carrot" to negotiators; the new Japanese leadership has not spelled out what will happen if a deal is not reached.
The ambitious target amounts to an emissions cut of about one-third from current levels in just 11 years, in a country that already uses energy efficiently.
The new government now has some serious thinking to do about how to turn rhetoric into reality.
Blog: Hatoyama's golden carrot
Analysts say the targets - announced by Mr Hatoyama at a climate change symposium in Tokyo on Monday - are more ambitious than those of many other industrialised nations.
They won praise from the climate change chief of the UN, which is recommending developed countries commit to a 25-40% reduction by 2020.
"With such a target, Japan will take on the leadership role that industrialised countries have agreed to take in climate change abatement," Yvo de Boer, head of the UN Climate Change Secretariat, told the conference.
Japan is the world's second-largest economy and fifth-largest emitter of greenhouse gases, which are a major contributor to climate change.
Correspondents say some Japanese business groups, including parts of the automotive industry, are expected to lobby against the plans.
The head of business lobby Keizai Doyukai (Japan Association of Corporate Executives) told the climate symposium the new government needed to spell out the policies in more detail.
"We basically welcome [the target], but we want to ask what policies and steps will be taken to achieve this 25% target," Masamitsu Sakurai told the forum, according to Reuters news agency.
Japan's new government envisages the plans will be achieved by bringing in emissions trading, renovating housing, subsidising solar panels and introducing low-energy technologies in cars.
Without mentioning China or India by name, Mr Hatoyama said: "We think developing countries are also required to make an effort to reduce greenhouse gases, as a global effort is needed on the issue of climate change," reports AFP news agency.
December's UN-backed climate talks in Copenhagen in Denmark will try to work out a deal on reducing emissions to succeed the current Kyoto Protocol, the first phase of which ends in 2012.
Japan has been under pressure to introduce tougher policies on climate change after its emissions rose last year to 16% above the Kyoto target, which it played a leading role in agreeing in 1997.