Boris Johnson and Matt Hancock (R) have tea with staff members during a Conservative Party general election campaign in Worksop, central England. File / AFP

British Prime Minister Boris Johnson became the first major world leader to test positive for the coronavirus on Friday as Spain suffered its deadliest day yet in a pandemic that has now killed more than 25,000 people around the world.

Britain's Health Secretary Matt Hancock also tested positive with mild symptoms.

The prime minister and health secretary both said they had “mild” symptoms. It came as London’s NHS scrambled to avert “armageddon” as a surge of cases in the capital was recorded.

 "Over the last 24 hours I have developed mild symptoms and tested positive for coronavirus," Johnson said.

"I am now self-isolating, but I will continue to lead the government’s response via video-conference as we fight this virus."

A Downing Street spokesman said Johnson, 55, experienced mild symptoms on Thursday — a day after he answered at the prime minister's weekly question-and-answer session in the chamber of the House of Commons.

"The prime minister was tested for coronavirus on the personal advice of England's Chief Medical Officer, Professor Chris Whitty," the spokesman said.

"The test was carried out in No 10 by NHS staff and the result of the test was positive," the spokesman said.

In a pair of grim milestones, an AFP tally showed the number of deaths hitting 25,066, most of them in Europe, and the United States overtook China as the country with the most coronavirus cases.

Early on Friday President Donald Trump insisted he had “much respect” for China’s response to COVID-19, tweeting: “China has been through much & has developed a strong understanding of the Virus. We are working closely together.”

And food suppliers have warned that fresh fruit and vegetables could become scarce in Europe due to disruption in the supply chain.

Economies were reeling — the head of the IMF said it was clear the world had entered a recession — and Africa's business powerhouse South Africa became the latest nation to start life under lockdown as it reported its first COVID-19 deaths.

Johnson, whose country has seen more than 14,000 declared coronavirus cases and 759 deaths, said he had developed mild symptoms over the previous 24 hours and was self-isolating after testing positive.

Spain reported that its death toll had hit 4,858 after 769 people died in 24 hours, a record one-day figure.

It was higher than the latest toll from hard-hit Italy, where experts said the epidemic could peak in the next few days but regional authorities warned the crisis was far from over.

The rate of new infections also appeared to be slowing in Spain, something officials cautiously described as "promising."

Europe has suffered the brunt of the coronavirus crisis in recent weeks, with millions across the continent on lockdown and the streets of Paris, Rome and Madrid eerily empty.

'She just had a cough'

In France — where nearly 1,700 people have died — the government announced it was extending its stay-at-home order until at least April 15.

"We find ourselves in a crisis that will last, in a health situation that will not improve any time soon," Prime Minister Edouard Philippe said.

The death of a 16-year-old girl from the virus has particularly shaken France, and shattered the belief of many young people that they are immune.

The girl's mother Sabine told AFP that Julie "just had a cough" at first but deteriorated quickly. She died on Wednesday, less than a week after showing her first symptoms.

"It's unbearable," Sabine said. "We were supposed to have a normal life."

'They didn't listen'

Focus was also turning to the United States, where the number of known infections reached almost 86,000, higher than both China and Italy.

In New York City, health workers are battling a surging toll of dead and infected at the US epicentre of the crisis, including an increasing number of younger patients.

"Now it's 50-year-olds, 40-year-olds, 30-year-olds," said one respiratory therapist at the Jewish Medical Center in Queens.

They "didn't listen about not going out or protecting themselves and washing their hands", he said.

"To watch somebody in their 30s die, it's hard. You can't have visitors. They're in the room by themselves on a ventilator. It's very depressing."