TOKYO - JAPAN'S economics minister, fiscal reformer Kaoru Yosano, said on Thursday he planned to challenge front runner Taro Aso as a candidate for the nation's next prime minister.

Mr Aso, 67, wants the government to spend more to support the faltering economy, the world's second-largest, but some lawmakers in the ruling Liberal Democratic Party (LDP) worry he could derail plans for fiscal reform.

Also gunning for the position, young conservative Nobuteru Ishihara, a son of Tokyo's outspoken governor. He said on Thursday he was ready to run for prime minister, calling for his party to challenge Mr Aso.

Mr Aso, a flamboyant former foreign minister who supports government spending to boost the troubled economy, is an early favourite to win the September 22 vote to take the reins of the Asian financial power.

The ruling Liberal Democratic Party (LDP) was left scrambling to find a replacement after Prime Minister Yasuo Fukuda abruptly quit on Monday following months of lacklustre poll ratings.

Mr Ishihara, 51, said he would seek the required backing of 20 lawmakers to run against Mr Aso, a three-time loser for the top job who is now the LDP secretary general, if Mr Aso goes unchallenged.

'I must resolve to start the work of collecting recommendations if nobody else runs,' Mr Ishihara told reporters. 'The time to make a decision is approaching.'

'I am on the opposite wing of secretary general Taro Aso. I have to ensure that there is a chance to speak on that stand,' Mr Ishihara said.

Mr Aso backs spending to revive the economy, which is now teetering on recession as inflation soars and demand for exports slow. Japan has the highest public debt of any developed nation.

Mr Ishihara served as the minister in charge of administrative reforms under popular premier Junichiro Koizumi, who spearheaded free-market policies in the world's second largest economy.

Ms Yuriko Koike, a former television anchorwoman, has also emerged as a possible contender and is backed by reformists close to Mr Koizumi. She has not publicly announced her candidacy.

Ms Koike, who has served as defence minister and environment minister, would be Japan's first female prime minister.

Mr Ishihara would be Japan's youngest prime minister in modern times. Former premier Shinzo Abe was 52 when he took office in 2006 and faced criticism that he was not ready for the job.

The LDP has been in power for all but 10 months since it was created in 1955. But it lost control of one house last year to the opposition, which has tried to block the LDP agenda in hopes of winning the next elections.

Mr Ishihara is the son of Tokyo Governor Shintaro Ishihara, who enjoys wide popularity for his strong-armed leadership but is despised by liberals for his sometimes crude remarks extolling Japan.

The younger Ishihara is a former television journalist who has also served as the LDP's policy board chief and transport minister. -- AFP