Abe has battled the disease ulcerative colitis for years while two recent hospital visits within a week 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.
He was scheduled to hold a news conference at 5:00 p.m. (0800 GMT).
The conservative member of parliament returned as prime minister for a rare second term in December 2012, pledging to revive growth with his “Abenomics” mix of hyper-easy monetary policy, fiscal spending and reforms.
On Monday, he surpassed a record for longest consecutive tenure as premier set by his great-uncle Eisaku Sato half a century ago.
Under fire for his handling of the coronavirus pandemic and scandals among party members, Abe has recently seen his support fall to one of the lowest levels of his nearly eight years in office.
He has beefed up military spending and expanded the role of the armed forces, but his dream of revising the pacifist constitution has failed due to divided public opinion.
He resigned abruptly as prime minister in 2007, citing ill-health after a year plagued by scandals in his cabinet and a huge election loss for his ruling party. He had since kept his illness in check with medicine that was not previously available.