Johnson said earlier in the day that he was in "good spirits" after being admitted to the hospital due to the coronavirus. These included a high temperature and a cough.
The prime minister's official spokesman said he remained in hospital "under observation", and described Russian reports that Mr Johnson had been placed on a ventilator as "disinformation".
He's conscious at this time and the move to the ICU was done as a precaution in the event Johnson should need ventilation to help him recover.
Johnson, who had been isolating in Downing Street after testing positive for the virus last month, was taken to St Thomas' Hospital in central London on Sunday night after the symptoms continued for a period longer than 10 days.
"The prime minister is in intensive care, being looked after by his medical team, receiving the very, very best care from the team at St Thomas'", Gove said.
As the de facto first secretary of state, Raab was asked by a reporter if he was ready to take charge if needed, said he was not going to get into hypotheticals.
"The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication."
Johnson's spokesman, James Slack, earlier refused to say what kind of tests Johnson was undergoing.
"While we have faced challenges before, this one is different", the queen said.
Former UK Prime Minister David Cameron, who went to school with Johnson, said in a tweet that he was thinking of Johnson and his family and that he was "in great hands, and we all want you safe, well and back in @10DowningStreet".
She said he is also likely to have an electrocardiogram to check his heart's function, as well as tests on his oxygen levels, white blood cell count, and liver and kidney function before he is released from hospital.
According to the office, Johnson's condition deteriorated on Monday afternoon. He first announced his positive test result on March 27.
But something important has changed, and he has felt it necessary to ask his foreign secretary to deputise for him where needs be. "He's being given regular updates on developments and he continues to lead the government".
The government said Monday that 51,608 people had been confirmed to have the coronavirus in Britain, 5,373 of whom have died.
President Donald Trump is among those to have sent good wishes to Johnson.
"Americans are all praying for his recovery", Trump said during a White House press briefing.
Responding to the news, Labour leader Sir Keir Starmer tweeted: "Terribly sad news".
Celebrities Who Suffer With Chronic Diseases
Now if that isn’t the epitome of what a BFF is, I don’t know what is! Selena continues to be on chronic medication. There is also a huge list of celebrities past like famous painter vincent Van Gogh who suffered with this disease.