Japan's new prime minister will be formally elected by parliament on 1 October following next week's leadership contest, a ruling party official said Wednesday.

Polls indicate that three frontrunners are emerging among the nine candidates to succeed Fumio Kishida as head of the Liberal Democratic Party (LDP) in the 27 September internal vote.

They are conservative economic security minister Sanae Takaichi, 63; former LDP secretary general Shigeru Ishiba, 67; and Shinjiro Koizumi, 43, son of former premier Junichiro Koizumi.

The conservative LDP -- which has governed almost uninterrupted for decades -- holds a majority in parliament, meaning the winner of the party election is essentially guaranteed to become premier.

Yasukazu Hamada, a LDP lawmaker in charge of parliament affairs, told his opposition counterpart that the party "plans to convene a parliament session on October 1" to elect the new prime minister, a LDP official told AFP.

The opposition party accepted the date, which will be formally announced by the government on Monday, media reports said.

Kishida, 67, whose three-year term was tarnished by scandals, voter anger over rising prices and sliding poll ratings, announced last month that he was stepping down.

In the leadership election, each of the LDP's 367 parliament members cast a vote, and another 367 votes will be determined based on the preferences of rank-and-file party members and supporters.

Polls by different Japanese media have put Takaichi, Ishiba and Koizumi in the lead, although this is no guarantee any of them will emerge as the eventual winner.

Koizumi would be Japan's youngest-ever premier while Takaichi, a vocal nationalist popular with the LDP's conservative wing, would be the country's first woman leader.

As a regular visitor to the Yasukuni shrine to Japan's war dead, which includes convicted war criminals -- her nomination would likely rile victims of Japan's wartime aggression such as China and South and North Korea.