Boris Johnson is beginning his first working day in self-isolation in Downing Street after meeting an MP who later tested positive for Covid-19.
The prime minister said he was “pinged” by NHS Test and Trace on Sunday and told to self-isolate, but remains well.
Mr Johnson on Thursday spent about 35 minutes with Tory MP Lee Anderson, who lost his sense of taste the next day.
It comes as the government prepares a policy relaunch after fierce internal rows and departures from No 10.
Downing Street said a series of “critical announcements” would this week detail Mr Johnson’s “ambitions for the United Kingdom”.
They follow the dramatic departure of the PM’s chief adviser Dominic Cummings last week.
Mr Johnson will chair “key Covid meetings” and work with Chancellor Rishi Sunak to devise the upcoming spending review with an aim to fulfil his promise to “build back better”.
The PM will now be working from his flat at Downing Street.
Mr Johnson wrote on Twitter on Sunday night: “Today I was notified by NHS Test and Trace that I must self-isolate as I have been in contact with someone who tested positive for COVID-19.
“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.”
In a WhatsApp message to Tory MPs seen by the BBC the PM added: “The good news is that NHS Test and Trace continues to improve. The bad news is that I have been pinged!”
He said he would observe self-isolation rules despite “following the guidance and socially distancing” during his meeting with Mr Anderson.
“It doesn’t matter that I feel fine – better than ever – or that my body is bursting with antibodies because I have already had the damn thing,” he added.
“The rules are the rules and they are there to stop the spread of the disease.”
In April, Mr Johnson spent three nights in intensive care after falling ill with coronavirus.
He later said it “could have gone either way” and thanked healthcare workers for saving his life.
It remains unclear what effect, if any, previously having the coronavirus has on a person’s immunity but experts think reinfection is likely to be rare, BBC health correspondent James Gallagher has reported.
On Thursday, Mr Anderson, the Conservative MP for Ashfield, posted a photo of himself with Mr Johnson at No 10 alongside the words: “Breakfast with the PM.”
On Sunday, he posted on his Facebook page to say he was self-isolating with his wife, who is clinically vulnerable, after they both tested positive.
Boris Johnson will now have to stay at home in No 10.
It means he will not be able to be in Parliament, though I’m told he will be working from Downing Street.
He does still intend to keep communicating with the country.
It was supposed to be a pretty big week for Boris Johnson – he is trying to reset his government after some factional fighting in his office over the last few days.
There are conversations taking place with the parliamentary authorities to see whether he can still contribute to the Commons.
I think it is fair to say this has not come at the best time for Mr Johnson: he has big decisions to make on Brexit and what happens when England’s lockdown ends on 2 December.
And it is also worth bearing in mind he was extremely ill with coronavirus earlier in the year and we do not know what getting the virus does for a person’s immunity.
Leader of the House of Commons Jacob Rees-Mogg said on Sunday evening he was urgently exploring how to “support additional virtual participation” in the chamber.
Such a move could allow more MPs, including Mr Johnson, to attend Commons’ debates virtually and possibly even Prime Minister’s Questions as he self-isolates.
Earlier this weekend, senior Tory MPs said Mr Cummings’ departure was a chance to “reset government” and a series of announcements are planned for this week.
A meeting between the PM and the Northern Research Group of backbench Tory MPs had been scheduled for Monday.
Officials also confirmed the government’s 10-point plan for a “green industrial revolution” would be published “to boost green jobs whilst invigorating plans to achieve net zero (emissions) by 2050”.
Elements of that already announced include a new £40m investment in green spaces across England and £160m for the wind turbine industry.
In addition, another week of negotiations over a post-Brexit trade deal with the EU are set to begin in Brussels.
The transition period, which followed Brexit in January, ends on 31 December by which time a deal needs to be agreed and approved by parliaments in the UK and EU.
Chief UK negotiator David Frost has said there had been some progress between the two sides but that considerable differences remain.
The UK government announced another 24,962 confirmed Covid cases on Sunday, as well as a further 168 deaths within 28 days of a positive test.