Boris Johnson is "feeling well" after having a "minor routine operation" this morning, his spokesman has said, with Deputy Prime Minister Dominic Raab stepping in while the PM recovers.
Mr Johnson was under general anesthetic for the operation relating to his sinuses and has been told he must rest until tomorrow.
His deputy Mr Raab is in charge of big government decisions until the prime minister returns, Number 10 said.
Mr Johnson's spokesman would not get into the exact nature of the operation or why it was necessary but did reveal it was carried out by the NHS and the prime minister was under general anesthetic.
The PMs official spokesperson said: "He went to hospital around 6am and the operation was carried out first thing this morning.
"He was back in Downing Street shortly after 10am."
Providing an update on the PM's condition on Monday afternoon, his spokesman said the operation "went well" and he is "pleased to be back in No 10".
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The prime minister will chair a meeting of his Cabinet on Tuesday morning, the spokesman added.
Asked who was in charge of the nuclear accounts, the spokesperson said the Deputy Prime Minister and Cabinet Secretary were aware of the procedure in advance and that Mr Johnson was under for a "relatively brief time".
He said: "Operationally, my understanding is there's a 24-hour period from when the operation takes place ... under which sort of significant decisions would be deferred to probably in the first instance the Deputy Prime Minister.
"Now we're not anticipating any of that being required currently, but we do have that capability."
It is not the first time Mr Raab has stepped in as acting prime minister.
He was asked take on many of the PM’s duties in April 2020 when Mr Johnson entered intensive care while suffering with coronavirus.
Mr Raab, who at the time was foreign secretary before being moved over his handling of the Afghanistan evacuation, said he was acting "at the prime minister's direction" while standing in.
With Mr Johnson conscious and at home following this morning's operation, it is likely Mr Raab would seek the PM's guidance before making any big decisions.