Nigerian President Muhammadu Buhari's latest visit to London has been met by outcry, with his critics enraged at his frequent trips abroad for medical treatment and the country's frail health care system.

Buhari, 78, has made several trips to London since he came to power in 2015 and the leader was reelected for another four-year term in 2019.

Although the nature of his ailment has never been made public, Buhari confessed during one of the trips that he had "never been so ill" and that he had received several blood transfusions.

His frequent trips abroad for medical treatment have brought criticism about the government's transparency over his illness and worries about leadership during some of his longer absences.

On his latest visit to the British capital on Tuesday, the presidency said the retired general, who was a military ruler in the 1980s, would be away for two weeks.

"He is due back in the country during the second week of April," his office said in a statement.

The main opposition Peoples Democratic Party (PDP) condemned the trip, saying it was an indication that Nigeria's health care system had failed.

"It is indeed worrisome that under President Buhari, even the hitherto highly rated State House Clinic, has become so moribund that it cannot provide a simple medical checkup service for Mr. President," the PDP said in a statement.

The party said Buhari's frequent trips were a drain on the treasury.

"The PDP is disturbed that while Mr. President jets out for medical treatment in well-equipped hospitals abroad, our hospitals and medical personnel are in very sorry situation while millions of our compatriots suffer," it said.

Both the local media and Nigerians on social media expressed their outrage over the latest trip.

Members of the Nigerian diaspora were planning protests in London against the president's visit, said activist Omoyele Sowore.

"We are totally appalled by it," he said of the latest medical visit. "He goes to London at the slightest provocation even when he has a headache."

The latest London trip came just hours before a threatened strike by doctors in government hospitals over inadequate pay and facilities.

Last year a third of Nigeria's doctors went on strike amid the COVID-19 pandemic demanding better pay and working conditions.

Buhari's health became a subject of debate ahead of the last election when the opposition claimed he was not physically fit to govern – but Buhari won a second term.

In 2017, he temporarily handed over power to his vice president to allay fears over the government's leadership. But his absence provoked calls for him to return or resign.

The president's state of health became a sensitive subject in Nigeria after former leader Umaru Musa Yar'Adua, died in 2010 and it emerged that his ailment had been kept secret for months.