Japan began searching for a new prime minister on Tuesday after Yasuo Fukuda became the second leader to abruptly resign in less than a year, threatening a further policy vacuum as the economy teeters on the brink of recession.
The frontrunner to become the 11th prime minister in 15 years is former foreign minister Taro Aso, 67, an outspoken nationalist popular with voters who was runner-up to Mr Fukuda in the race for party chief last year.
Mr Aso said he thought he was a suitable candidate in a party vote, which Kyodo news agency said was expected on 22 September.
Mr Fukuda, 72, had been struggling to cope with a divided parliament where opposition parties have the power to delay legislation, and his sudden exit raised questions about his conservative party's ability to cling to power or even hold together after ruling Japan for most of the past 53 years.
The bespectacled Fukuda, a moderate conservative who favours close ties with Japan's Asian neighbours, only took office in September last year after his predecessor, Shinzo Abe, similarly quit after 12 months in office.
An economic relief plan unveiled on Friday that included a promise of income tax cuts and about $US16.5 billion in extra spending this year to ease the pain of rising prices, failed to revive Mr Fukuda's popularity among voters.
His resignation does not automatically mean an early election, ahead of the due date of September next year.
However, whoever the ruling Liberal Democratic Party picks as its leader, and thus the next prime minister, might choose an early poll to take advantage of any rise in public support.
A complete deadlock in parliament could also force the prime minister to call an election reluctantly. The ruling coalition is almost certain to lose seats, if not its majority, but voters say they want a turn to pick their government.