"The president wishes to assure all Nigerians that there is no cause for worry," his office says.
PIC: President Buhari's previous trip to London was in January. (AFP)
Nigeria's President Muhammadu Buhari headed to London on Sunday for what his office said was a "follow-up medical consultation" with his doctors, after weeks of mounting concern about his health.
Presidency spokesman Femi Adesina said the 74-year-old doctors would determine how long he stayed in the British capital, where he spent nearly two months undergoing treatment from mid-January.
"The president wishes to assure all Nigerians that there is no cause for worry," Adesina said in a statement posted on the presidency's Facebook page and Twitter feed.
"He is very grateful for the prayers and good wishes of the people, and hopes they would continue to pray for the peace and unity of the nation."
President Buhari is pictured in this December 13, 2016 photo arriving at a hotel in Banjul to meet with Gambian president-elect with three other African heads of state. (AFP)
Buhari's previous trip to London in January was billed by his office as a 10-day holiday combining "routine medical check-ups" but it was extended from early February and he only returned in early March.
Missed cabinet meetings
The former military ruler then contradicted his aides insistence that he was "hale and hearty", telling his senior ministers he had undergone blood transfusions and had never been as sick in his life.
This time, no timeframe was given for his absence.
"The length of the president's stay in London will be determined by the doctors. Government will continue to function normally under the able leadership of the Vice-President (Yemi Osinbajo)," Adesina said.
Parliament had been informed, as required by the constitution, he added.
Buhari had not been seen in public for several weeks until last Friday, when he attended weekly prayers at the presidential villa, after missing the last three cabinet meetings and other engagements.
Here, the Nigerian leader sits among the 82 rescued Chibok girls during a reception ceremony at the Presidential Villa in Abuja, on Sunday. (PGDBA&HND Mass Communication/Sunday Aghaeze)
Aides have said he was working from home and had undergone a "long period of treatment" in London.
Buhari has been under pressure to disclose the nature of his illness but has said he would have to have follow-up treatment in London.
Earlier on Sunday, he was pictured sitting in an armchair at his residence, dressed in white traditional robes, looking painfully gaunt, surrounded by 82 recently released Chibok schoolgirls.
Adesina said Buhari would have left earlier on Sunday but wanted to meet the students, who were kidnapped by Boko Haram Islamists in northeast Nigeria in April 2014, causing worldwide outrage.
The health of Nigeria's president has been a sensitive issue after Umaru Musa Yar'Adua fell ill and died in office in 2010 from a long-standing, but undisclosed, kidney complaint.
His long absence abroad and death triggered months of political turmoil.