LONDON (Reuters) - British Prime Minister Boris Johnson was admitted to hospital for tests on Sunday after suffering persistent coronavirus symptoms 10 days after testing positive for the virus, though Downing Street said he remained in charge of the government.

Johnson, who was isolating in Downing Street after testing positive last month, still had a high temperature and so his doctors felt he should go to an undisclosed hospital for tests in what the government said was a “precautionary step”.

“On the advice of his doctor, the prime minister has tonight been admitted to hospital for tests,” his Downing Street office said in a statement. Johnson is expected to stay overnight.

“This is a precautionary step, as the prime minister continues to have persistent symptoms of coronavirus ten days after testing positive for the virus,” the statement added.

News of his hospitalisation came only after an hour after Queen Elizabeth delivered a rallying call to the British public saying they would overcome the coronavirus outbreak if they stayed resolute.

Johnson, 55, on March 27 became the first leader of a major power to announce that he had tested positive. He went into isolation at an apartment in Downing Street and said on Friday he was staying there as he still had a high temperature.

Downing Street underscored that this was not an emergency admission and that Johnson remained in charge of the government. Foreign Secretary Dominic Raab will chair the government’s emergency COVID-19 meeting on Monday, a source said.

With only an unwieldy collection of sometimes ancient and contradictory precedents to go by, there is no simple, formally-enshrined “Plan B” or succession scenario should the prime minister become incapacitated.

The pound briefly fell 0.4% vs the U.S. dollar to $1.2215 before trimming some losses to trade at $1.2230, down 0.3%.

Queen Elizabeth had been informed of Johnson’s admission to hospital, Buckingham Palace said. It made no further comment.

U.S. President Donald Trump said Johnson was a “strong man” as he passed on his nation’s best wishes.

“All Americans are praying for him,” Trump told a news conference. “He’s a friend of mine, he’s a great gentleman and a great leader, and as you know he went to the hospital today but I’m hopeful and sure that he’s going to be fine.”

CORONAVIRUS IN THE UK

Johnson, the face of the 2016 Brexit campaign, won a resounding election victory in December before leading the United Kingdom out of the European Union on Jan. 31.

But he has faced criticism in the United Kingdom for initially approving a much more modest response to the novel coronavirus outbreak than other major European leaders, telling a news conference on March 3 that he had been shaking hands with coronavirus patients.

He then changed tack when scientific projections showed a quarter of a million people could die in the United Kingdom.

Johnson effectively shuttered the world’s fifth-largest economy, advising people to stay at home and the elderly or infirm to isolate themselves for weeks.

FILE PHOTO: Prime Minister Boris Johnson speaks during a news conference on the ongoing situation with the coronavirus disease (COVID-19) in London, Britain March 22, 2020. Ian Vogler/Pool via REUTERS/File Photo

But the virus has penetrated the British government.

Johnson and his health minister tested positive last month and his chief medical adviser also self isolated. Johnson’s pregnant 32-year-old fiancee, Carrie Symonds, also had symptoms but said on Saturday she was feeling better.

From an apartment above Number 11 Downing Street, and with food brought to his door, Johnson continued to lead the government’s response and chaired meetings via video conference.

He has posted a series of video messages on Twitter since then, initially appearing in a suit and tie but in the latest post on Friday, he appeared weary, sitting in a chair with his shirt open at the neck.

“Although I’m feeling better and I’ve done my seven days of isolation, alas I still have one of the symptoms, a minor symptom, I still have a temperature,” he said.

Britain’s National Health Service guidelines stipulate that those who suspect they have coronavirus should not leave home.

Under a section headlined “What to do if you need medical help while self-isolating”, it says: “If you need medical help for any reason, do not go to places like a GP (family doctor) surgery, pharmacy or hospital.”

Health officials said on Sunday the UK death toll from the coronavirus had risen by 621 to 4,934.

Downing Street refused to say what tests Johnson was to have in hospital, but experts said a person of the prime minister’s age with COVID-19 symptoms after 10 days would be likely be assessed for their oxygen levels, lung, liver and heart functions, and undergo an electrocardiogram heart check.

Doctors managing COVID-19 patients had reported that more men than women had serious problems and patients who were overweight or had previous health issues were at a higher risk.

Earlier this year, Johnson said he needed to lose weight.

“We don’t know exactly why the PM has gone to hospital except we have been told he is having tests,” said Derek Hill, professor of medical imaging science at University College London.

“Many people attending hospital with COVID-19 have difficulty breathing. Some people are rapidly discharged. Some others can quickly deteriorate and need help breathing. We have no reason to believe the PM needs such help.”