By Yuka Obayashi, Nobuhiro Kubo and Leika Kihara

TOKYO (Reuters) -Japanese Prime Minister Shigeru Ishiba resigned on Sunday, ushering in a potentially lengthy period of policy uncertainty at a shaky moment for the world's fourth-largest economy.

Having just ironed out final details of a trade deal with the United States to lower President Donald Trump's punishing tariffs, Ishiba, 68, told a press conference he must take responsibility for a series of bruising election losses.

Since coming to power less than a year ago, the unlikely premier has overseen his ruling coalition lose its majorities in elections for both houses of parliament amid voter anger over rising living costs.

He instructed his Liberal Democratic Party - which has ruled Japan for almost all of the post-war period - to

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