"Over the course of [Monday] 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."
It continued: "The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication."
Johnson, 55 was admitted to hospital in London with "persistent symptoms" on Sunday evening.
Earlier on Monday, he tweeted that he was in "good spirits".
The UK’s Foreign Secretary Dominic Raab has been asked by the prime minister to deputise "where necessary".
However, experts say his move to the intensive care unit meant the UK leader was "extremely sick", but a spokesman said Mr Johnson was "conscious" and the move was a precaution.
He was initially taken to hospital for routine tests after testing positive for coronavirus on March 26. His symptoms included a high temperature and a cough.
At the time of his diagnosis, Johnson released a short video on his Twitter account saying he had "developed mild symptoms" in the previous 24 hours.
A spokesman said Russian media reports that Mr Johnson was on a ventilator was "misinformation" but refused to answer when asked directly whether the Prime Minister had pneumonia.