The Prime Minister’s condition has worsened, Downing Street has said.
Mr Johnson was admitted to St Thomas' Hospital in London on Sunday night - 10 days after testing positive for coronavirus - due to his continuing "persistent" symptoms of COVID-19.
He was initially admitted for further tests but Mr Johnson's health has since deteriorated, Number 10 said.
The Prime Minister is understood to have experienced breathing difficulties on Monday afternoon and was given oxygen.
He is conscious and has been moved to intensive care as a precaution should he require ventilation.
Last night, on the advice of my doctor, I went into 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.— Boris Johnson #StayHomeSaveLives (@BorisJohnson) April 6, 2020
Politicans from across the spectrum in Oxfordshire have wished the Prime Minister well on Twitter this evening.
Lib Dem MP Layla Moran, who represents Oxford West and Abingdon, penned: "There are no political parties tonight. Only bated breath and good wishes."
John Howell, the MP for Henley and Robert Courts, who represents Witney, said their thoughts are with Boris Johnson and his family and Banbury MP Victoria Prentis also echoed a similar message.
Oxfordshire's newest MP David Johnston Retweeted a post by Sajid Javid which read 'Boris is one of the strongest people I know. Thinking of him, Carrie and his family tonight. Get well soon, the country needs you.'
While, Anneliese Dodds, the new shadow chancellor and MP for Oxford East re-tweeted a post from the new Leader of the Labour Party who described it as "terribly sad news."
Foreign Secretary Dominic Raab has been asked to deputise the Prime Minister "where necessary", a request that was made before Mr Johnson was moved to intensive care.
Meanwhile, the Prime Minister's pregnant fiancee Carrie Symonds revealed at the weekend she spent a week in bed with coronavirus symptoms.
There are no political parties tonight. Only bated breath and good wishes.— Layla Moran 🔶 (@LaylaMoran) April 6, 2020
My thoughts and prayers are with Boris and his family.— John Howell (@JHowellUK) April 6, 2020
All my thoughts & prayers are with @BorisJohnson & @carriesymonds tonight.— Robert Courts MP #StayHomeSaveLives (@robertcourts) April 6, 2020
The boss is the toughest of fighters & that is what we all need right now. The fervent support and wishes of millions of people are with him tonight.
Keep fighting PM. #backboris #GetWellBoris