U.K. Prime Minister Boris Johnson has been placed in intensive care as he battles to defeat the coronavirus that has plagued him for more than 10 days.

Johnson, 55, was admitted to hospital Sunday night for routine testing, but remained there on Monday. According to a Downing Street spokesperson, Johnson has asked Foreign Secretary Dominic Raab to deputize while he is infirmed. Reports of Johnson’s admission to intensive care surfaced just past 8 p.m. local time.

A statement reads: “Since Sunday evening, the prime minister has been under the care of doctors at St Thomas’ Hospital, in London, after being admitted with persistent symptoms of coronavirus. 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.”

Downing Street has said Johnson is “receiving excellent care, and thanks all NHS staff for their hard work and dedication.”

Johnson’s intensive care treatment comes just hours after he took to Twitter on Monday afternoon to explain his situation: “Last night, on the advice of my doctor, I went to hospital for some routine tests as I’m still experiencing coronavirus symptoms. I’m in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe.”

Johnson also thanked “the brilliant NHS staff taking care of me and others in this difficult situation. You are the best of Britain.”

In a press briefing Monday afternoon, the prime minister’s spokesperson said he remained in hospital for observation, but refused to say whether there has been a diagnosis of pneumonia. The spokesperson also kiboshed Russian reports that Johnson had been placed on a ventilator as “disinformation.”

The U.K. does not have an official deputy role for someone to step in if the prime minister’s incapacitation is prolonged; however, at the onset of the coronavirus outbreak, Foreign Secretary Raab was appointed Johnson’s “designated survivor” and will take over duties.

As news of Johnson’s hospitalization has spread, a number of political leaders have expressed well wishes for the prime minister.

France’s president Emmanuel Macron shared on Twitter that he gives his “full support” to Johnson, his family and the British people.

Meanwhile, former prime minister David Cameron also took to Twitter to write, “You are in great hands and we all want you safe, well and back in 10 Downing Street.”

U.S. President Donald Trump is among those who sent his wishes to Mr. Johnson following Sunday’s hospitalization. “All Americans are praying for him. He’s a great friend of mine, a great gentleman and a great leader,” said Trump, adding he was sure Johnson would be fine because he is “a strong person”.

Johnson’s condition comes as the U.K. struggles to control its coronavirus outbreak, which has so far claimed 5,373 lives. Close to 52,000 have so far tested positive.

Johnson’s partner Carrie Symonds, who is pregnant with the couple’s first child, tweeted Saturday that she has “spent the past week in bed with the main symptoms” of coronavirus. “I haven’t needed to be tested, and after seven days of rest, I feel stronger and I’m on the mend.”

News of the prime minister’s hospitalization on Sunday came just hours after a rare televised address to the nation by Queen Elizabeth, rallying the U.K. against the coronavirus.

“We should take comfort that while we may have more still to endure, better days will return: we will be with our friends again; we will be with our families again; we will meet again,” said the monarch.