The Prime Minister, who was admitted to intensive care with coronavirus in April, is “well” and does not have any symptoms of the virus, according to a Number 10 spokesman.
Mr Johnson was advised to self-isolate by NHS Test and Trace after attending a 35-minute meeting with a small group of MPs in Downing Street on Thursday morning, including MP for Ashfield Lee Anderson.
Mr Anderson said on Facebook that he began experiencing symptoms of Covid-19 on Friday and, after being tested on Saturday, received a positive result on Sunday morning.
“The Prime Minister has today been notified by NHS Test and Trace that he is required to self-isolate as a contact of someone who has tested positive for Covid-19,” the Number 10 spokesman said.
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“The Prime Minister will follow the rules and is self-isolating.
“He will carry on working from Downing Street, including on leading the Government’s response to the coronavirus pandemic.”
The spokesman added: “The PM is well and does not have any symptoms of Covid-19.”
Mr Johnson tested positive for Covid-19 at the end of March and was admitted to a London NHS hospital on April 5, which Downing Street said was a “precautionary step” as his symptoms persisted.
He then spent three nights in St Thomas’ Hospital’s intensive care unit, later revealing it “could have gone either way” and thanked NHS staff for saving his life.
Writing on Twitter on Sunday evening, the Prime Minister said he would continue to “lead the government’s pandemic response” from self-isolation in Downing Street.
“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,” he said.
“I have no symptoms, but am following the rules and will be working from No10 as I continue to lead the government’s pandemic response.”
Mr Anderson said in a Facebook post on Sunday evening that he had lost his sense of taste on Friday and went for a test the following day.
“On Friday I lost my sense of taste at the same time my wife had a bad headache. I had no cough, no fever and felt well,” Mr Anderson said.
“We both had a test on Saturday and the result came in Sunday morning. My wife and I both tested positive.
“I feel absolutely fine and my biggest concern is my wife who is in the shielded group. But we are both feeling good.”
On Thursday, Mr Anderson posted a picture of himself with the Prime Minister after the breakfast meeting in Downing Street – neither were wearing face coverings and appeared to be less than two metres apart.
Downing Street insisted that Number 10 is a Covid-secure workplace and every step is taken to minimise infection, but due to the length of the meeting Mr Johnson was advised to self-isolate.
It follows a week which saw Mr Johnson’s administration rocked by political drama with his close aides, chief adviser Dominic Cummings and communications director Lee Cain, exiting their posts.
Only hours before it was announced that the Prime Minister had been told to self-isolate, details were released of plans for “critical announcements” that would set a “clear signal of his ongoing ambitions for the United Kingdom”.
Downing Street said the measures ahead would include plans to tackle the coronavirus outbreak, moves for “levelling up” across the country, and education and environmental investment initiatives.
While a spokesman said the Prime Minister will continue working from Number 10, he was expected to lead a Downing Street press conference this week.
On Monday, the Prime Minister was expected to meet Tory MPs from the newly formed Northern Research Group (NRG), which was set up to press the case for “levelling up” northern England.
The self-isolation period means he will also miss facing Labour leader Sir Keir Starmer at Prime Minister’s Questions (PMQs) on Wednesday.