Boris Johnson needed a "significant level of specialist treatment" at the worst points of his battle against coronavirus, according to his spokesman.
The PM left hospital over the weekend, having previously been in intensive care at one point of his stay.
Mr Johnson has now tested negative for Covid-19, the spokesman confirmed, and is recovering at Chequers.
“Now, clearly, if a person is moved to intensive care, that means that they are requiring a significant level of specialist treatment," the spokesman said.
“So, I think it was clear from the moment the Prime Minister was moved to intensive care that his condition had worsened.”
He would not go into further details of just how ill the Prime Minister had been.
Asked about the Prime Minister being tested before he left hospital, Mr Johnson’s spokesman said: “I’m told it is standard practice in hospital to do that.
“And the results came back negative.”
It is unclear when the PM will return to work though at present he is said to be following medical advice by not doing so, while he will be taking further guidance before going back.
Mr Johnson said yesterday that there was "no question" that the NHS saved his life as he fought the virus.
“It’s hard to find the words to express my debt – but before I come to that, I want to thank everyone in the entire UK for the effort and the sacrifice you have made and are making," he said in a video statement.
Dominic Raab, the Foreign Secretary and First Secretary of State, has been filling in for Mr Johnson when necessary.
Listen to The Leader: Coronavirus Daily podcast
The spokesman said that Mr Johnson had spoken to Mr Raab, stating: “He spoke to the First Secretary of State over the course of the weekend.
“The Prime Minister is focusing on his recovery and he is not currently carrying out Government work.”
