Boris Johnson has spent the night in intensive care after his coronavirus symptoms worsened, with overall charge of the government handed to Dominic Raab, the foreign secretary.

In an unprecedented development during a public health crisis, the prime minister was moved to intensive care at St Thomas’ hospital, London, at 7pm on Monday as a precaution in case he needed ventilation, it was said. He remained conscious on Monday night.

A No 10 spokesman said: “Over the course of this afternoon, the condition of the prime minister has worsened and, on the advice of his medical team, he has been moved to the intensive care unit at the hospital.

Play Video 1:50 Donald Trump says Americans are praying for 'resolute' Boris Johnson – video

“The prime minister has asked foreign secretary Dominic Raab, who is the first secretary of state, to deputise for him where necessary. The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication.”

Intensive care beds are reserved for those who are very ill, and in the case of Covid-19, often those who need to be put on a mechanical ventilator to help them breathe. Since first being admitted to St Thomas’ on Sunday evening, Johnson, 55, is understood to have received oxygen through a non-invasive mask only.

His pregnant fiancee, Carrie Symonds – herself recovering from coronavirus symptoms – was said to be deeply anxious about Johnson’s condition.

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News of his hospitalisation, 11 days after he developed a cough and high temperature, caused national and international concern, with world leaders from US president Donald Trump to French president Emmanuel Macron sending their best wishes and support for Johnson’s recovery.

Trump said: “We’re very saddened to hear that [Johnson] was taken into intensive care this afternoon, a little while ago, and Americans are all praying for his recovery. He’s been a really good friend. He’s been really something very special: strong resolute, doesn’t quit, doesn’t give up.”

He said he had asked two companies offering treatment for coronavirus to contact London “immediately”. “They’ve really advanced therapeutics ... and they have arrived in London already, the London office has whatever they need. We’ll see if we can be of help. We’ve contacted all of Boris’s doctors, and we’ll see what is going to take place, but they are ready to go.”

Shortly after the announcement, Raab appeared on the airwaves with an attempt to reassure the public, saying government business would continue. He told the BBC there was an “incredibly strong team spirit” behind Johnson and that ministers would focus on delivering the plans he had put in place.

He said the prime minister was in safe hands and receiving excellent care and vowed that the government would bring the whole country through the pandemic.

Buckingham Palace said of the Queen: “Her Majesty has been kept informed by No 10.”

The decision to hand over power to Raab, 46, was taken after concern in the cabinet and among MPs that Johnson should have been resting instead of working. It also comes amid suggestions that the seriousness of the prime minster’s coronavirus symptoms – including a persistent cough and high temperature - were being underplayed by himself and his aides.

At midday on Monday, Downing Street was still insisting that the prime minister was able to read his red box of government papers, and at the daily 5pm press conference, Raab claimed Johnson was still fully in charge and issuing instructions.

No 10 had insisted earlier on Monday that Johnson was only in hospital as a “precautionary measure” and for “routine tests”, having said up until his admission on Sunday evening that he was suffering from mild symptoms.

Claims that his condition had not deteriorated came under increased scrutiny after it emerged that a bed at St Thomas’ was being prepared for Johnson as early as Thursday last week. He spent Sunday night on an empty wing that is normally used for elective procedures, a source said, before being moved to intensive care on Monday evening.

Before his move to intensive care, the Guardian was told of frustration among ministers over Johnson remaining in charge for so long rather than resting, even after he had received oxygen treatment having failed to shake off Covid-19 symptoms for 11 days.

It came as the UK death toll from coronavirus exceeded 5,000 for the first time, rising by 439 to 5,373. The daily reported increase fell for the second day in a row, but a Department of Health source cautioned that this can be due to lower reporting at weekends and said the trend needs to be lower during the week to be a cause for optimism.

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At the daily Downing Street press conference at 5pm, Raab said it remained the case that Johnson was “in charge, he is leading and giving instructions as and when required”.

However, he later admitted not having spoken to Johnson since Saturday – the day before the prime minister’s admission to hospital on medical advice.

No 10 insisted it had been “transparent throughout” about Johnson’s medical condition, despite having claimed up until the point of his hospital admission that his symptoms were “mild”. His spokesman dropped that description on Monday, saying instead that his cough and temperature were “persistent”.

Two ministers, James Duddridge and Nadine Dorries, suggested Johnson needed time to recuperate. Duddridge said it was important for the prime minister to now “rest, look after yourself and let the others do the heavy [lifting]”.

But Iain Duncan Smith, the former Tory leader and a friend of the prime minister, said he had good reasons for not wanting to step away until now.

He told the BBC: “His first thought has not actually been for himself, it has been in a sense for the British people. He doesn’t want the British people to feel in any way overly concerned and worried. He wants them to be calm, which goes to the nature of the individual he is.”

Duncan Smith also attempted to calm fears around Johnson’s admission, saying the public “shouldn’t over-read the going into intensive care”.

“I am certain that the doctors will have done this to make sure that they can stabilise him completely so they can treat whatever he may have. As I say, he may have a strong secondary infection which needs to be treated and treated rather urgently,” he said.

After the prime minister’s hospital admission, Raab led the daily coronavirus morning meeting, known internally as the “war cabinet”. But several cabinet sources said there was a worry that Johnson’s refusal to let go of power had been causing impotence at the top of government.

Dominic Cummings, the prime minister’s most senior adviser, is also isolating with coronavirus symptoms and has not yet returned to work.

While the prime minister has been in self-isolation for over a week, reported tensions between Rishi Sunak, the chancellor, and Matt Hancock, the health secretary, emerged over the competing demands of the health impacts of coronavirus and its effects on the economy. Tory sources insisted any problems were largely at the level of officials.

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Meanwhile, No 10 and the Department of Health have been frustrated with both NHS England, for allegedly failing to share data quickly enough at “sit-rep” [situation reports] meetings on the state of services, and Public Health England, for not agreeing to look to private firms for outside testing capacity quickly enough.

Chris Whitty, the chief medical officer, also said it was “too early” and a “mistake” to have a “serious discussion about all the things we need to do step-by-step to move to the next phase of managing this” without the UK having seen a peak in cases.

He also stressed the government would need to balance many factors in deciding whether to ease the lockdown. These include people dying directly from the virus itself, dying indirectly if the NHS became overwhelmed with cases, the effects of postponing other types of healthcare because of coronavirus, and the long-term health impact on people due to social and economic reasons.