Japanese Prime Minister Shigeru Ishiba will announce his resignation by the end of next month, following the Liberal Democratic Party's defeat in the upper house election, The Mainichi, a newspaper, reported on Wednesday.

The report came shortly after Ishiba and US President Donald Trump announced that the two countries have reached a trade agreement to reduce the "reciprocal" tariff rate from 25 to 15 percent.

Although Ishiba had initially signaled his intention to remain in office despite the election defeat, mounting pressure from within his party — including calls for his resignation and criticism of his leadership — could accelerate the timing of his departure.

According to the newspaper, Ishiba is expected to meet soon with former prime ministers Taro Aso, Yoshihide Suga and Fumio Kishida to discuss his political future.

"It is unusual for a sitting prime minister to meet with multiple former prime ministers at the same time," the local media said.