Mr. Johnson, 55, went into isolation after experiencing mild symptoms of illness, his office said Friday.
British Prime Minister Boris Johnson announced on Friday that he is self-isolating after testing positive for COVID-19.
A Downing Street spokesperson said in a statement that Johnson, whose partner Carrie Symonds is pregnant, experienced mild symptoms on Thursday and was tested for COVID-19 on the personal advice of England's chief medical officer.
The government confirmed this week that if Johnson was incapacitated, Foreign Secretary Dominic Raab would temporarily assume the role of prime minister.