Boris Johnson is "stable" after spending the night in intensive care following a worsening of his coronavirus symptoms, Downing Street has said.
The Prime Minister's official spokesman said he had been receiving "standard oxygen treatment" but had not required any other assistance in breathing.
Mr Johnson was transferred to the intensive care unit at St Thomas' Hospital, London, on Monday evening, after his condition deteriorated.
His spokesman said the move was a "precautionary step" in case he needed to be put on a ventilator.
"The Prime Minister has been stable overnight and remains in good spirits," the spokesman said.
"He is receiving standard oxygen treatment and breathing without any other assistance.
"He has not required mechanical ventilation or non-invasive respiratory support."
Mr Johnson was originally admitted to St Thomas' on Sunday on the advice of his doctor after continuing to display symptoms of cough and high temperature ten days after testing positive for the virus.
The speed of the Prime Minister's decline has caused palpable shock at Westminster after his symptoms were previously described as "mild".
His spokesman, however, rejected claims that No 10 had sought to hide the seriousness of his condition.
"We have been fully frank with you throughout," the spokesman told reporters.
"We have issued you with regular updates on the Prime Minister's health.
"His condition worsened yesterday afternoon. A decision was taken that he needed to be moved to an intensive care unit at around 7pm.
"We informed you all as soon as was practically possible. We have a commitment to be as transparent as we can be throughout this process."