UK Prime Minister Boris Johnson began a third day in intensive care on Wednesday battling the coronavirus, which has struck at the heart of the British government, infected more than 55 000 people across the country and killed nearly 6 200.

"I understand the prime minister is in a stable condition, he's comfortable and in good spirits," Edward Argar, a junior health minister, told Sky News.

"He has in the past had some oxygen but he's not on ventilation."

The Times reported Johnson's persistently high temperature had fallen, while the Daily Telegraph said he was being cared for by one of Britain's leading lung doctors.

'A fighter'

Newspapers urged Britons to keep their stricken leader at the forefront of their minds, with the country in lockdown to try to stem the spread of Covid-19 in its third week.

"He stayed at work for you... now pray at home for him," The Sun tabloid splashed across its front page. "Boris 'will pull through'," said the Daily Express.

Deputising for Johnson, Foreign Secretary Dominic Raab called him "a fighter" and predicted "he'll be back, leading us through this crisis in short order".

Johnson, 55, is the most high-profile government leader to become infected with COVID-19 and messages of support flooded in from across Britain and the world.

He was admitted to intensive care on Monday evening after spending Sunday night in hospital following concerns he still had a cough and high temperature 10 days after being diagnosed with Covid-19.

His transfer to intensive care is unprecedented for a prime minister during a national emergency.

For many people, it brought home the seriousness of the disease that has so far seen 6 159 deaths in Britain, with a record 786 more reported in a daily update on Tuesday.

Despite the record daily death toll, there was more encouraging news with the number of new daily cases remaining at a roughly stable 3 643.