Shinzo Abe, Japan's longest-serving prime minister, says he's resigning because a chronic illness has resurfaced.

Concerns about Abe's health began this summer and grew this month when he visited a Tokyo hospital two weeks in a row for unspecified health checkups.

Abe has acknowledged having ulcerative colitis since he was a teenager and has said the condition was controlled with treatment.

Abe, whose term ends in September 2021, is expected to stay on until a new party leader is elected and formally approved by the parliament.

In a country once known for its short-tenured prime ministers, the departure marks the end of an unusual era of stability that saw the Japanese leader strike up strong ties with President Donald Trump even as Abe's ultra-nationalism riled the Koreas and China.

While he pulled Japan out of recession, the economy has been battered anew by the coronavirus pandemic, and Abe has failed to achieve his cherished goal to formally rewrite the US-drafted pacifist constitution because of poor public support.

Abe is a political blue blood who was groomed to follow in the footsteps of his grandfather, former Prime Minister Nobusuke Kishi. His political rhetoric often focused on making Japan a normal and beautiful nation with a stronger military and bigger role in international affiars.

Hiroshige Seko, a party secretary general for the Upper House of the parliament, confirmed that Abe told party executives he is resigning as prime minister.

Seko said Abe said he decided to resign in order not to cause trouble. Abe is scheduled to hold a news conference later Friday.

Concerns about Abe's chronic health issues, simmering since earlier this summer, intensified this month when he visited a Tokyo hospital two weeks in a row for unspecified health checkups.

Abe, whose term ends in September 2021, is expected to stay on until a new party leader is elected and formally approved by the parliament.

Abe became Japan's youngest prime minister in 2006, at age 52, but his overly nationalistic first stint abruptly ended in disappointment a year later because of his health.

In December 2012, Abe returned to power, prioritizing economic measures over his nationalist agenda. He won six national elections and built a rock-solid grip on power, bolstering Japan's defense role and capability and its security alliance with the US. He also stepped up patriotic education at schools and raised Japan's international profile.

Abe on Monday became Japan's longest serving prime minister by consecutive days in office, eclipsing the record of Eisaku Sato, his great-uncle, who served 2,798 days from 1964 to 1972.

But his second hospital visit Monday has accelerated speculation and political maneuvering toward a post-Abe regime.

Abe has acknowledged having ulcerative colitis since he was a teenager and has said the condition was controlled with treatment. He has not made clear if that condition is related to his recent health issues or hospital visits.

Ulcerative colitis causes inflammation and sometimes polyps in the bowels. People with the condition can have a normal life expectancy but serious cases can involve life-threatening complications.

After his recent hospital visits were reported, top officials from Abe's Cabinet and the ruling party said Abe was overworked and badly needed rest.

His health concerns added to speculation that Abe's days in office were numbered; his support ratings were already at their lowest levels due to his handling of the coronavirus pandemic and its severe impact on the economy, on top of a stream of political scandals.

There are a slew of politicians eager to replace Abe.

Shigeru Ishiba, a 63-year-old hawkish former defense minister and Abe's archrival, is a favourite next leader in media surveys, though he is less popular within the ruling party.

A low-key former foreign minister, Fumio Kishida, Defence Minister Taro Kono, Chief Cabinet Secretary Yoshihide Suga, and economic revitalisation minister Yasutoshi Nishimura, who is in charge of coronavirus measures, are widely speculated in Japanese media as potential successors.

Abe, who repeatedly faced public criticism for his handling of the coronavirus outbreak, was often upstaged by Tokyo Gov. Yuriko Koike, a former ruling party conservative who's seen as a potential prime minister candidate by some. But she would have to first be elected to parliament to be in the running for the top job.

The end of Abe's scandal-laden first stint as prime minister was the beginning of six years of annual leadership change, remembered as an era of revolving door politics that lacked stability and long-term policies.

When he returned to office in 2012, Abe vowed to revitalise the nation and get its economy out of its deflationary doldrums with his Abenomics formula, which combines fiscal stimulus, monetary easing and structural reforms.

Abe's unpopular coronavirus measures sent his support ratings plummeting before his health issues.

Perhaps Abe's biggest failure was his inability to fulfill a long-cherished goal of his grandfather to formally rewrite the US-drafted pacifist constitution. Abe and his ultra-conservative supporters see the US-drafted constitution as a humiliating legacy of Japan's WWII defeat.

He was also unable to achieve his goal of settling several unfinished wartime legacies, including normalizing ties with North Korea, settling island disputes with neighbours and signing a peace treaty with Russia.