Yusuf, who was rushed to Nairobi a month ago suffering from a chest complaint, arrived in the Ethiopian capital on Thursday from the government stronghold of Baidoa in south-central Somalia.
The 73-year-old president received a liver transplant nearly 14 years ago and has had to go abroad for specialist treatment on numerous occasions. He went to London for scheduled tests last month.
"This morning the president landed safe and sound in London," Yusuf Mohamed Ismail, the Somali ambassador to the U.N. European office in Geneva, said by telephone.
"He is back in London to receive part of the results of the medical tests he underwent and to undergo any new ones if necessary."
Ethiopian officials confirmed Yusuf’s departure.
Since the 1990s, Yusuf’s health has been under scrutiny. However, those close to him say he is strong and can handle the stress of Somalia’s anarchic politics.
His interim government, with Ethiopian military backing, has fought Islamist insurgents in the capital Mogadishu for the past year in a conflict that has cost 6,500 lives. (Reporting by Tsegaye Tadesse in Addis Ababa; writing by Bryson Hull; editing by Andrew Dobbie)