Japanese Emperor Akihito, who was taken to hospital last Sunday with a bronchial infection and was due to be discharged on Friday, will remain in hospital for at least another day after his fever returned, Kyodo news agency reported.
Akihito, who will turn 78 next month, had initial clearance to return to his palace, but will stay at the University of Tokyo Hospital until Saturday or later after his cough worsened and his fever went up, the agency said.
Akihito, who ascended to the throne after the death of his father Hirohito in 1989, has spent much of the past two decades working to close the gap between the imperial family and the people.
Five days after a devastating earthquake and tsunami struck Japan's northeast in March, Akihito made a rare public televised address and in April travelled to the disaster-struck area with Empress Michiko, 77.
Akihito had surgery for prostate cancer in 2003 and suffered stress-related health issues in late 2008, including irregular pulse and stomach bleeding.
The following year, the royal agency said he would cut back on official duties such as speeches and meeting foreign dignitaries.
(Reporting by Tomasz Janowski; Editing by Nick Macfie)