Japan's ruling coalition collapsed on Friday after its junior partner Komeito quit, raising doubts about whether conservative Sanae Takaichi will become the country's first female prime minister.

The announcement came less than a week after Takaichi became the Liberal Democratic Party's fifth leader in as many years, tasked with winning back disaffected voters.

What is the slush fund scandal?

The main reason given by Komeito party chief Tetsuo Saito was what he called the LDP's failure to tighten rules on the funding of political parties.

This follows a scandal within the LDP that emerged last year involving dodgy payments of millions of dollars related to ticket sales for fundraising events.

The revelations cost then-Prime Minister Fumio Kishida his job, led to several LDP factions being dissolved and to voters deserting the party at elections.

Takaichi caused particular consternation to Komeito by appointing Koichi Hagiuda, who was embroiled in the scandal, to a senior party post, reports said.

"For the Komeito party, which champions public integrity, it is difficult to explain this to its supporters and those working hard on the election campaign," said Hidehiro Yamamoto, politics and sociology professor at the University of Tsukuba.

"To them, it feels like they are losing elections with the LDP. There is little benefit to the coalition (for Komeito)," he said.

What else doesn't Komeito like?

Komeito began in the 1950s as the political wing of the Soka Gakkai, a large lay organization affiliated with a major Buddhist sect.

Many members have voiced anxiety about Takaichi, who has promoted staunchly conservative policies and has been a regular visitor to the war-linked Yasukuni Shrine.

The shrine in Tokyo honors all Japanese killed in conflict, including convicted war criminals.

The last visit to Yasukuni by a serving prime minister — by Takaichi's mentor, the late Shinzo Abe — in 2013 prompted fury from China and South Korea.

What now?

The LDP-Komeito coalition has governed nearly continuously since 1999 but, after poor election results, is now in a minority in both houses of parliament.

This has made passing legislation difficult, requiring opposition support on a case-by-case basis.

Media reports had suggested that Takaichi wanted to expand the coalition to include the Democratic Party for the People (DPP) since its economic policies have some overlap with hers.

However, DPP head Yuichiro Tamaki expressed his support for Komeito on Friday and its "extremely strong resolve to bring an end to the issue of politics and money."

The LDP and the DPP together would still fall short of the 233 seats needed for a majority in the Lower House. The LDP has 196 seats, the DPP 27 and Komeito 24.

Can Takaichi still become PM?

Takaichi needs a majority in her favor to be appointed by parliament, which appears difficult given the numbers.

However, she is helped by other parties needing to agree on an alternative candidate with more support — something that also looks unlikely.

More probable is that all the parties will each put forward their own leader in a first round of voting, and that Takaichi then comes through in a two-way runoff.

"But if the opposition blocs unite to choose their own candidate, it means a change of government," said Sadafumi Kawato, professor emeritus of the University of Tokyo.