Japan's main opposition Thursday decided not to run candidates in the upcoming election, effectively joining forces with a new party formed by popular Tokyo governor Yuriko Koike in a bid to topple Prime Minister Shinzo Abe.

The move capped a stunning few days on Japan's sleepy political scene that has seen the calling of a snap election, the creation of Koike's dynamic new "Party of Hope" and the effective unification of opposition to Abe's conservative government.

The centre-left Democratic Party (DP) approved its leader Seiji Maehara's proposal not to field candidates in the October 22 poll. Members should instead apply to represent Koike's new group, Maehara said.

"We must stop the Abe government and achieve a change of administration," said Maehara, a political veteran and former foreign minister.

Maehara said he would stay as chief of the DP but Thursday's decision effectively marked the death of the party, which has been wallowing in poor support ratings.

"No matter how good the product is, it is no use if it can't find a buyer," he told reporters.

The DP and its allies already held fewer than 90 seats in the lower house of parliament, less than one-third of the 287-strong force of Abe's conservative Liberal Democratic Party (LDP).

It is a far cry from when the DP swept to power in 2009, ousting the long-dominant LDP.

But the DP's spell in government was brief and it was booted from office in late 2012 by a resurgent Abe-led LDP following a series of mis-steps and policy flip-flops.

The party also came under fire for its handling of the tsunami-triggered Fukushima nuclear disaster in 2011.

According to a poll in business daily Nikkei less than a week ago, 44 percent of Japanese planed to vote for the LDP, while only eight percent favoured the main opposition.

But the emergence of Koike's "Party of Hope" has transformed the political landscape and new polls show Abe's lead has been cut dramatically. 

Koike, 65, who became Tokyo governor in a landslide last year, has said her party aspires to offer an alternative to the long-governing LDP and a break from old-school politics and links to vested interests.

Japanese voters will go to the polls on October 22 after Abe called a snap election, apparently in a bid to capitalise on the weak state of the DP.