Abe said he had been able to keep the illness, ulcerative colitis, under control for nearly eight years into his tenure, but he started to feel fatigue from mid-July and was diagnosed with a relapse earlier this month.

"As I am no longer in a condition to respond confidently to the mandate of the people, I have decided I should not stay in the prime minister's post," he said at a press conference.

Abe will not name an acting prime minister and will serve until the next leader is chosen. Asked who should succeed him, the premier refrained from commenting.