Japan's Defence Minister Tomomi Inada announces her resignation during a news conference at the Defence Ministry in Tokyo, Japan July 28, 2017. [Photo/Agencies]

Inada, 58, an Abe protege who shares his conservative views ,had already expected to be replaced in a likely cabinet reshuffle next week that Abe hopes will help repair his ratings.

Support for the prime minister has sunk below 30 percent in some polls, due to scandals over suspected cronyism and a view among many voters that he and his aides took them for granted.

Speaking to reporters after Inada announced her resignation at a separate news conference, Abe apologised "to the people from my heart".

He said Foreign Minister Fumio Kishida would add the defence portfolio to his duties, to eliminate any gap at a time when Japan faces tough security challenges.

Abe, however, had drawn fire from both ruling and opposition party lawmakers for retaining Inada despite her missteps and perceived incompetence.

"He should have thrown Inada under the bus long ago … doing so on the eve of a cabinet reshuffle only looks like desperation," said Jeffrey Kingston, director of Asian Studiesat Temple University Japan.

The resignation coincided with a report of an investigation into suspicions that defence ministry officials tried to hide logs showing worsening security in South Sudan, where Japanese troops joined in a U.S.-led peacekeeping operation.

Critics said troop deployment in the dangerous environment violated conditions set for such activities in line with Japan's pacifist constitution. No Japanese troops have died in combat since World War Two and the growing chaos in South Sudan fuelled concern.

Abe, who returned to power in December 2012 promising to revive the economy and bolster defence, had until recently been seen as likely to win a third term when his tenure ends in September 2018, putting him on track to be Japan's longest-serving prime minister.

But the ratings plunge is making it more likely that LDP rivals will challenge him for the top party post - and the premiership.

The main opposition Democratic Party, however, has failed to capitalise on Abe's troubles.

Its leader, Renho, who goes by one name, said on Thursday she would resign as the best way to try to repair her party's image, tarnished by persistent bickering and memories of its rocky 2009-2012 rule.

Financial markets are taking the political developments in stride. "Until it gets to the point where Abe is going t oresign, this is not affecting markets at all," said Mitsushige Ando, chief fund manager at Ichiyoshi Asset Management.

Reuters