Abuja - Nigeriaan President Muhammadu Buhari is to prolong his stay in London pending the result of health tests, his spokesperson said on Sunday in a statement which will do little to quash rumours he is unwell.Femi Adesina said Buhari had written to the National Assembly on Sunday "informing them of his desire to extend his leave in order to complete and receive the results of a series of tests recommended by his doctors".The 74-year-old, who has been dogged by speculation about whether he is physically fit for office even before he took power in May 2015, had been due to return to Abuja on Sunday after 10 days' holiday.The presidency previously announced Buhari would combine time off in the British capital with routine health checks but has been forced to counter rumours he was gravely ill or even dead.The former army general, who headed a military regime in the 1980s, was last in London in June 2016 for treatment to what was said at the time to be a persistent inner ear infection.Adesina said on Sunday Buhari had been "advised to complete the test cycle before returning". The lower and upper houses of parliament had been informed, he added.Vice-President Yemi Osinbajo has been deputising for Buhari in his absence.No time-frame for Buhari's return was given, nor any details about the medical tests he had undergone.The health of Nigeria's president is a sensitive issue. In 2010, president Musa Umaru Yar'Adua died in office from a long-standing, but previously undisclosed, kidney complaint. His initial illness and treatment in hospital abroad triggered months of political uncertainty. His deputy, Goodluck Jonathan, took over on Yar'Adua's death.General Sani Abacha, who headed a military regime in the 1990s, also died in office, officially from a heart attack. But rumours persist he met a more lurid end.