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 after they were released by Boko Haram Islamist militants.

The girls were handed over on Saturday in exchange for five Boko Haram suspects after negotiations, sources have told the BBC.

A list of the girls' names has been published, but it is not clear if all the parents have been formally notified, whether or not their girls are among those released.

They were from a group of 276 abducted from their school in north-eastern Nigeria. About 113 of the girls are still missing, along with hundreds of other people kidnapped by the militant group.

Nigerians are particularly sensitive to the health of their president after then President Umaru Yar'Adua sought medical treatment in Saudi Arabia in 2009.

His failure to hand power to the vice-president and the lack of information about his condition led to widespread anxiety. He died in office in 2010.

When Mr Buhari returned from London in March, he 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.

Concerns over his health were rekindled after he missed the last three cabinet meetings, prompting civil society activists to urge him to return to London for further treatment.

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 people thought.

Buhari's unhealthy start to 2017

19 January - Leaves for UK on "medical vacation"

5 February - Asks parliament to extend medical leave

10 March - Returns home but does not resume work immediately

26 April - Misses second cabinet meeting and is "working from home"

28 April - Misses Friday prayers

3 May - Misses third cabinet meeting

5 May - Appears at Friday prayers in Abuja

7 May - Travels to UK for further treatment