British Prime Minister Boris Johnson has spent a second night in intensive care and was in a stable condition after receiving oxygen support for COVID-19 complications, as his foreign minister directs the country's battle against the outbreak.

Johnson, who tested positive nearly two weeks ago, was taken to St Thomas' hospital in central London on Sunday evening as he had a persistent high temperature and cough, but his condition deteriorated on Monday and he was transferred to an intensive care unit.

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The 55-year-old British leader received oxygen support but was not put on a ventilator and his designated deputy, Foreign Secretary Dominic Raab, said he would soon be back at the helm as the world faces one of the gravest public health crisis in a century.

"He is comfortable, he's stable, he's in good spirits," Edward Argar, a junior health minister, said on Wednesday. "While he's had oxygen, he hasn't been on a ventilator."

As Johnson fought the novel coronavirus in hospital, the United Kingdom was entering what scientists said was the deadliest phase of the outbreak and grappling with the question of when to lift the lockdown.

Inside the British government, ministers were debating how long the world's fifth-largest economy could afford to be shut down, and the lasting implications of one of the most stringent sets of emergency controls in peacetime history.

The total number of hospital deaths from COVID-19 rose by 786 to 6,159 as of 16:00 GMT on April 6, the latest publicly available death toll, though just 213,181 people out of the United Kingdom's population of around 68 million have been tested.

Johnson has delegated some authority to Raab, who was appointed foreign minister less than a year ago, although any major decisions - such as when to lift the lockdown - would need the blessing of Johnson's cabinet.

"I’m confident he’ll pull through because if there’s one thing I know about this prime minister, he’s a fighter, and he’ll be back at the helm leading us through this crisis in short order," Raab said on Tuesday.

British finance minister Rishi Sunak, a 39-year-old former Goldman Sachs banker, would deputise should Raab become incapacitated.