Japan's next prime minister will be maverick politician Shigeru Ishiba after the ruling Liberal Democratic Party (LDP) elected him the new leader following the news Prime Minister Fumio Kishida would step down.

Mr Ishiba, a veteran politician and former defence minister, won the top job after a runoff against Sanae Takaichi in an internal ballot after none of the original nine hopefuls won enough support to win the ballot outright.

Had Ms Takaichi won, she would have become Japan's first female prime minister.

The vote was sparked after Mr Kishida announced he would no longer seek the top job at the scheduled party meeting, following months of low approval ratings and a donations scandal that embroiled high-ranking politicians.

Before the final vote among LDP politicians, Mr Ishiba vowed to clean up the party's image.

"We will put an end to the widespread distrust in the LDP," he said.

Mr Ishiba, 67, has tried and failed to secure the top job many times.

He is known as a maverick, speaking against the party when he feels necessary, which has made him popular among voters.

"He appears on television media quite a lot to give very frank and honest opinions, including criticism of his own government, and that has made him popular with voters," said Jeff Hall, an expert in politics at Kanda University.

He represents a regional part of Japan suffering population decline and has spoken about the need to help all of Japan, and not just the big cities.

"He has this sort of idealistic focus on helping every part of Japan, and he is also very much a policy expert-kind of politician.

"He loves to talk about defence policy, natural disaster relief policies."

But his criticism of the party, including leaving the LDP before, has hurt him in the past, with some fellow LDP members calling him a traitor.

Ishiba beat out would-be first female PM

Mr Ishiba beat out Ms Takaichi, 63, the economic security minister and a hardline conservative who has praised Margaret Thatcher as a role model.

A protege of former prime minister, the late Shinzo Abe, Ms Takaichi talked about stimulating the economy and maintaining ultra-low interest rates.

But there was concern her conservative views were out of line with the electorate.

"She's extremely conservative on social views, far to the right of most Japanese voters, on things like gender, on same-sex marriage, on the issue of whether or not women should be allowed to have a separate family name when they get married," Mr Hall said.

She also advocated visiting the Yasukuni Shrine, which honours Japan's war dead, including convicted war criminals.

Her position would infuriate China and South Korea, a country Japan has tried to build relations with.

"She is also very, very hawkish towards China," Mr Hall said.

One of the initial frontrunners in the race was Shinjiro Koizumi, son of former prime minister Junichiro Koizumi.

At 42, Mr Koizumi would have become Japan's youngest prime minister were he successful, but a series of gaffs led many to conclude he was too inexperienced.

"There are a lot of jokes and memes that depict him as an air-headed person who doesn't really have much substance to him."

How the votes were counted

The LDP leadership is elected from a 50-50 split between party members, of which there are just under 1.1 million, and its 368 members of parliament, the Diet.

If no candidate wins enough support from the initial vote, a runoff is held that is only open to the members of parliament.

This means a politician less popular with the public can be elected, as factional heads will get the ultimate say.

"Kishida was a classic example of a leader selected not because of his appeal to voters, but because factions within his party wanted to block another politician from becoming PM," said Mr Hall.

In this runoff, Ms Takaichi was more popular in the initial vote, achieving 72 votes from Diet members while Mr Ishiba won 46.

But among party members, the vote was much closer, with Ms Takaichi beating her rival by one vote.

Snap election expected

Japan's political system is renowned for its relatively high rotation of prime ministers, yet one where the electorate continues to re-elect the same political party.

The LDP has ruled Japan almost uninterrupted in its post-war years.

It lost majority for only a few months in 1993 and was out of government between 2009 and 2013, losing to the centre-left Democratic Party of Japan.

But those years were marred by internal fighting and the the centre-left party failed to deliver on key policies, prompting voters to return the LDP, viewing them as the more experienced party to run the country.

In recent years, the government has been riddled with scandal. The most explosive were revelations senior members of government failed for years to declare donations.

While Mr Kishida wasn't directly involved in the donations scandal, voters saw him as unable to fix the problem, and his voter approval ratings tanked.

Despite this, the LDP is still expected to win the next general election.

"The LDP, at least, has the experienced people and the know-how to run a government without messing things up, at least in an opinion of many people," Mr Hall said.

"The various scandals of the LDP are, of course, abhorrent, and people are very annoyed by them, but a lot of them are petty corruption scandals and various other scandals that don't cause the country to grind to a halt, or they don't involve breaking major promises to voters."

The main opposition party, the Constitutional Democratic Party of Japan, has appointed former prime minister Yoshihiko Noda as its leader, to try and unite the opposition forces and appear as a reliable pair of hands.

Mr Ishiba is expected to call a snap election, meaning Japan will be returning to the polls in late October or November.