Boris Johnson is self-isolating after an MP he had a meeting with later tested positive for COVID-19.
The prime minister said he had been notified by NHS Test and Trace that he has to self-isolate as he has been in contact with someone who has tested positive.
Mr Johnson tweeted: "I have no symptoms, but am following the rules and will be working from No 10 as I continue to lead the government's pandemic response."
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Mr Johnson had a meeting lasting about 35 minutes with some MPs in Number 10 on Thursday morning, including Lee Anderson.
Mr Anderson then developed symptoms for COVID-19 and tested positive.
The MP for Ashfield and Eastwood, north of Nottingham, posted a picture of himself on Facebook standing apart from Mr Johnson after the meeting on Thursday. Neither were wearing face masks.
He said: "Breakfast with the PM. This morning I met with the PM at Number 10.
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"I was there with my wish list for Ashfield and Eastwood. Investment is coming, you have my word on that."
Andy Carter, another Conservative MP who was at the same meeting, said he has also been contacted by NHS Test and Trace and will now self-isolate.
The PM spent three nights in intensive care at St Thomas' Hospital in central London in April after testing positive for COVID-19 on 27 March.
He is hoping to continue to speak to the country during his self-isolation period, which should end on 26 November.
Mr Johnson's self-isolation comes at the end of a week of turmoil in which his director of communications, Lee Cain, resigned, followed by his top aide Dominic Cummings on Friday.
Next week was meant to be a fresh start as the final days of Brexit negotiations with the EU are due to start.
Now, he will have to take part in what could be the most important days of the talks via video call.