Nigeria's President Muhammadu Buhari is due to travel to London "for a scheduled medical follow-up", a statement from his office says.

This is the second time this year that he will have travelled to see doctors in the UK's capital. He was in London for a fortnight in April.

The 78-year-old has made several trips to the UK for medical reasons throughout his presidency, which began in 2015, but the nature of his illness has never been disclosed.

"He is due back in the country during the second week of July," the president's office says.

Mr Buhari has been criticised in the past for travelling abroad for treatment, especially as the country's public health service remains severely under funded.