The 74-year-old leader, who has been out of the country for two weeks, was expected to return to Abuja on Sunday.

An official statement said he had been advised by doctors to await the results of a series of tests.

However, it did not say how much extra time would be needed.

There was also no mention of what the medical checks were for.

Analysts say that Mr Buhari's extended absence could further erode confidence in his administration which is already under pressure due to a weak economy and the ongoing conflict with Boko Haram Islamist militants in the north-east of the country .

Vice-president Yemi Osinbajo is taking on presidential responsibilities while the president is abroad.

It is the second time in less than a year that Mr Buhari has sought medical assistance overseas. Last June, he was spent nearly two weeks, again in London, for treatment for an ear infection.