Boris Johnson is recovering at home after a minor operation for sinus problems on Monday morning.
Deputy Prime Minister Dominic Raab is nominally in charge of the nation’s security for 24 hours as Johnson recovers from the effects of the operation at an NHS hospital under general anaesthetic.
A Downing Street spokesman said: "The Prime Minister’s operation was a minor, routine operation in relation to his sinuses."
Responsibility was passed to Dominic Raab while the PM underwent the scheduled operation at an undisclosed NHS hospital.
The official said: "We follow standard processes for things like this, the deputy Prime Minister, who was aware in advance and the Cabinet Secretary, was also aware.”
Downing Street would not comment further on the routine operation or whether the Prime Minister had been on a waiting list for the procedure.
The Prime Minister is scheduled to chair the Cabinet on Tuesday morning, before which any urgent decisions would be taken by the deputy Prime Minister.
He is due to travel to Rwanda later this week to participate in the Commonwealth Heads of Government Meeting (CHOGM) in Kigali and onwards to the G7 Summit at Schloss Elmau, Germany, and the NATO Summit in Madrid, Spain.
Asked how the Prime Minister was feeling, the spokesman said he had not spoken to him since his return to Downing Street but that he was resting at home.
The timing of Johnson's return to work would depend on how he feels, but he was planning to chair Tuesday morning's Cabinet meeting, according to the spokesperson.
Asked who was in charge of the UK nuclear accounts during the procedure, the spokesperson said Deputy Prime Minister Dominic Raab and Cabinet Secretary Simon Case were aware in advance and that Mr Johnson was under for a "relatively brief time".
Johnson's sinus issue was not understood to be related to his Covid illness in 2020.
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