Boris Johnson has said he is feeling “great” on his first working day in self-isolation in Downing Street – after meeting an MP who later tested positive for Covid-19.
In a video on Monday, he urged others to “follow the rules” and self-isolate if contacted by NHS Test and Trace.
The prime minister said he was “pinged” by the scheme on Sunday.
On Thursday Mr Johnson spent about 35 minutes with Tory MP Lee Anderson, who lost his sense of taste the next day.
The PM’s period of isolation begins 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”.
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’s self-isolation comes seven months after he fell ill with coronavirus, and spent three nights in intensive care.
He later said it “could have gone either way” and thanked healthcare workers for saving his life.
The new policy plans follow the dramatic departure of the PM’s chief adviser Dominic Cummings last week.
Speaking on BBC Breakfast, Health Secretary Matt Hancock said the prime minister’s self-isolation would make no difference to the amount of work he would be able to do “driving forward the agenda”.
Asked if the PM and Mr Anderson followed social distancing rules during their meeting, he said there were rules “around Downing Street being a Covid-secure workplace”.
He added: “The central point is that it doesn’t matter who you are, if you are contacted by NHS Test and Trace and told to self-isolate that is what you must do.
“You could be the prime minister, you can be anybody in this country – we have the same rules and we all abide by them.”
In the video, the PM added: “The good news is that NHS Test and Trace is working ever-more efficiently, but the bad news is that they’ve pinged me and I’ve got to self-isolate because someone I was in contact with a few days ago has developed Covid.
“It doesn’t matter that we were all doing social distancing, it doesn’t matter that I’m fit as a butcher’s dog, feel great.
“And actually, it doesn’t matter that I’ve had the disease and I’m bursting with antibodies. We’ve got to interrupt the spread of the disease.”
Mr Johnson said he had a “high heart” that the country was getting on top of the virus, with rapid speed testing and the prospect of a vaccine providing reasons for encouragement.
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.
Mr Johnson wrote on Twitter on Sunday night that he had been notified by NHS Test and Trace that he must self-isolate as he had been in contact with someone who tested positive, and he would be working from No 10.
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.
Meanwhile, in other coronavirus developments:
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.