By making clear that she will take the helm of the party, Koike is apparently trying to help the party create momentum for the July 2 Tokyo metropolitan assembly election.

Koike will formally announce her decision to assume the party's top post at its rally on Thursday.

"For realizing Tomin First's ambition and carrying out its policies, I decided to be chief of the party, on top of Tokyo governor," Koike told reporters at her office.

Koike is still a member of the Liberal Democratic Party, the ruling party in national politics and the largest force in the Tokyo assembly. But she is at loggerheads with the party.

Koike scored an overwhelming victory over an LDP-backed candidate and other rivals in the July 2016 Tokyo gubernatorial election, in which she decided to run despite opposition from the party.

Koike said she has already asked the LDP about what to do with her party membership, adding that she will wait for the party's decision on the matter.

Initially, Koike declined to hold any post at Tomin First, to concentrate on her job as governor. At the end of April, however, she assumed the post of special adviser to the party. Currently, Kazusa Noda, secretary of Koike, is serving as head of the party.

For the upcoming Tokyo assembly election, Tomin First has so far decided to endorse 45 people as its candidates. The party hopes that its members and others supportive of Koike will secure a majority in the 127-seat assembly. (Jiji Press)