An extra edition of a daily newspaper reporting on Japan's Prime Minister Yoshihide Suga deciding not to run for the Liberal Democratic Party (LDP) presidential election is distributed in Tokyo's Ginza district on Friday. Photo: AFP

Former Japanese internal affairs minister Sanae Takaichi is set to announce her bid for ruling party leadership on Wednesday, her office said, which if successful would see her become Japan's first female prime minister.

Takaichi has the backing of former prime minister Shinzo Abe, local media said.

Japan's Liberal Democratic Party (LDP) will hold a leadership election on September 29, after Prime Minister Yoshihide Suga announced Friday he was stepping down. The winner of the vote is all but assured to be Japan's next prime minister.

So far only soft-spoken former foreign minister Fumio Kishida has announced his candidacy, but popular COVID-19 vaccination minister Taro Kono and Takaichi have signaled their ambition to run.

Takaichi, 60, became the first female internal affairs minister in the second Abe administration in 2014.

But even as local media have said influential Abe has thrown his support behind Takaichi helping her obtain the 20 lawmaker backers needed to run in the leadership election, she has ranked poorly in popularity ratings, which could hamper her chances.

Grass-roots LDP members will vote in the leadership election along with the party's members of parliament, and whoever wins will lead the party to the lower house election that must be held by November 28, making public appeal an important factor in choosing the new leader.

Takaichi has said she wanted to work on issues left unresolved by previous administrations.

She has opposed allowing married couples to keep separate surnames, to the disappointment of promoters of women's rights. Takaichi is due to speak at 4 pm. Kishida is giving a policy speech at 10 am.