Nigeria’s President Muhammadu Buhari is seeking fresh treatment in London for an undisclosed illness.
His health has been a major cause of concern in a country where there are fears that a power vacuum could affect its recovery from recession.
In a brief message, the president said “there is no cause for worry”.
Mr Buhari, 74, has left Vice-President Yemi Osinbajo in charge, just as he did when he left for London in January for seven weeks of medical leave.
Mr Osinbajo was widely praised at the time for his performance as acting president.
The exact length of the president’s stay in London “will be determined by the doctors”, a statement from the presidency said.
But, it added, “government will continue to function normally under the able leadership of the vice-president”.
His last official act before leaving for London was to meet the 82 schoolgirls kidnapped in Chibok in 2014 who had just been released by Boko Haram Islamist militants.
In 2009, then President Umaru Yar’Adua left the country for treatment in Saudi Arabia. His failure to hand over power to the vice-president led to widespread anxiety.
Mr Yar’Adua died in office in 2010.
On his return from London in March, Mr Buhari said he had never felt “so sick” as he had when he was being treated and warned that he may have to undergo further medical checks.
Mr Buhari’s health has become a major issue in Nigeria after he missed three cabinet meetings in a row.
His appearance at Friday prayers last week was the first time he had been seen in public for two weeks.
His aides said he had been resting and working from home and the president’s wife Aisha Buhari said he was not as sick as was being perceived.