Boris Johnson had a “routine” operation on the NHS in London shortly after 6 am GMT.
He was unconscious for a “relatively brief period of time” after being placed under general anesthesia, according to No10.
The operation is understood to relate to a sinus condition.
With Johnson recovering from his treatment, Deputy Prime Minister Dominic Raab has been asked to step up.
Any “significant” decisions on national security or other matters that may arise will be made by the Esher and Walton MP in conjunction with the Cabinet Secretary.
Johnson is still expected to chair the Cabinet tomorrow and will travel to a Commonwealth summit in Rwanda later this week as planned.
Raab was last asked to deputize for Johnson in the spring of 2020 when he was rushed to hospital suffering from Covid.
The Prime Minister spent a number of days in intensive care where he received oxygen treatment.
On that occasion, Raab was in charge for 20 days while his boss recuperated.