The prime minister's chief adviser Dominic Cummings is in self-isolation after showing coronavirus symptoms, No 10 has confirmed.
He started developing symptoms over the weekend and will be staying in contact with the rest of the Downing Street team during his quarantine period.
His health minister, Matt Hancock, subsequently tested positive, while the government's chief medical adviser, Chris Whitty, and Scottish Secretary Alister Jack are self-isolating after experiencing symptoms.
Mr Cummings was filmed running across Downing Street shortly after the prime minister posted a video message announcing he had tested positive for coronavirus.
Britain's initial response to containing the spread of the highly contagious respiratory infection was modest in comparison to European countries such as Italy.
However, Mr Johnson imposed stringent controls after projections showed a quarter of a million people in the UK could die.
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The prime minister insisted he would still be able to work as he self-isolates in 11 Downing Street through "the wizardry of modern technology".
The government's chief scientific adviser Sir Patrick Vallance said the prime minister was already using "digital tools" to manage meetings.
Mr Johnson said in a video message that the UK will get through the crisis "together", as he praised the 750,000 volunteers who have offered to assist the health service.
His remarks came after England's deputy chief medical officer, Dr Jenny Harries, said it could be six months or more before the UK gets back to normal from the pandemic.
Housing, Communities and Local Government Secretary Robert Jenrick warned additional lockdown measures could be enforced if current restrictions are deemed to be insufficient, or if Britons fail to comply with them.