Abe called a snap election on Monday as regional tensions remained at fever pitch over North Korea, coupled with Koike's declaration that she would head a new conservative grouping to be called the Hope Party.

"I will dissolve the House of Representatives on the 28th of September," said Abe who last month reshuffled his cabinet as ratings rebounded in the wake of cronyism scandals.

Read more: Japan's Abe using North Korea's threats to boost election chances

The head of Abe's junior coalition partner, Natsuo Yamaguchi, said he understood the election would be held on October 22.

Opposition lawmakers had said there was no need for new elections. A survey last weekend by the Nikkei business daily newspaper indicated that 44 percent of voters intended to vote for Abe's Liberal Democratic Party (LDP). He has been in power for five years,

The main opposition Democratic Party, which held power between 2009 and 2012, drew only 8 percent support. Another 8 percent was attributed to Koike.

A Kyodo news agency survey showed the LDP garnering a far lower 27.7 percent, with 42.2 percent of the electorate undecided.

Conservative rival

Tokyo's governor, whose city grouping won a municipal assembly landslide in July, said her new party would be conservative and would push for transparency in government, women's advancement and elimination of nuclear energy.

"Our ideal is to proceed free of special interests," Koike, a former LDP member, told a news conference on Monday, just hours before Abe's announcement.

Defections from LDP

Abe, on a nationally televised news conference, said he needed a mandate to shift some revenues from a planned future tax hike to social spending such as education.

Over the weekend, a junior LDP cabinet minister, Mineyuki Fukuda, said he too would leave the ruling party to stand for election with Koike's new group.

ipj/se (Reuters, AP)