“During his normal annual checkup, tests showed he needed a longer period of rest, necessitating the president staying longer than originally planned,” the presidency said in a statement on Tuesday.
“President Buhari wishes to reassure Nigerians that there is no cause for worry,” it said, without saying when Buhari might return.
Buhari, 74, left for Britain for medical leave a month ago, putting Vice President Yemi Osinbajo, 59, in charge during his absence. He had originally planned to stay 10 days.
Officials have refused to disclose his illness, triggering fierce speculation in Nigerian media and on social media. That speculation has been fuelled by a previous illness, when he spent nearly two weeks in London last June treating an ear infection.
Buhari’s predecessor, Goodluck Jonathan, was sworn in after the death in 2010 of President Umaru Yar’Adua. His illness created a power vacuum that was only filled by Jonathan, his vice president, after three months of political infighting.
Officials say the situation is different because Buhari has given Osinbajo full powers as acting president.
On Monday, the central bank devalued the naira foreign exchange rate for retail customers. Buhari has repeatedly spoken against a devaluation.