Shinzo Abe takes off his face mask at the start of a press conference at the prime minister's official residence in Tokyo on Friday. AP

Prime Minister Shinzo Abe, Japan's longest-serving premier, announced his resignation because of poor health on Friday, ending a stint at the helm of the world's third-biggest economy in which he sought to revive growth and bolster its defences.

"I have decided that I will step down as prime minister, with the belief that I cannot continue being prime minister if I do not have the confidence that I can carry out the job entrusted to me by the people," Abe, 65, told a news conference.

He said he had decided to step down now to avoid a political vacuum as the country copes with its novel coronavirus outbreak.

"I apologize from the bottom of my heart that despite all of the support from the Japanese people, I am leaving the post with one full year left in my term and in the midst of various policies and coronavirus," Abe said. He similarly quit in 2007 after one year as premier, citing illness.

It was the second time Abe has resigned as prime minister because of poor health.

Abe has battled the disease ulcerative colitis for years and two recent hospital visits within a week had fanned questions on whether he could stay in the job until the end of his term as ruling party leader, and hence, premier, in September 2021.

Speculation about the prime minister's health has been swirling for weeks but reached fever pitch in recent days after he made two separate trips to the hospital for unspecified medical checks. 

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Abe took three days of holidays this month and on August 17 made an unannounced hospital visit, staying there for more than seven hours for medical checks.

He made a second visit to the same hospital a week later for additional tests and said at the time that he intended to continue in the job. 

The resignation will trigger a leadership race in the ruling Liberal Democratic Party (LDP) - most likely in two or three weeks - and the winner must be formally elected in parliament. The new party leader will hold the post for the rest of Abe's term. 

On Monday, Abe surpassed a record for longest consecutive tenure as premier set by his great-uncle Eisaku Sato half a century ago.

Agence France-Presse