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.
Boris Johnson's spokesman said on Monday that Mr Cummings had not yet been tested.
On Friday, the prime minister became the first leader of a major world power to test positive for the virus.
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.
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.
He said teams are working "around the clock" to deliver equipment to frontline NHS staff - with 170 million masks, 42.8 million gloves and 2.3 million pairs of eye protectors delivered to 58,000 healthcare facilities so far.
In a two-and-a-half minute message, Mr Johnson said "we have 20,000 NHS staff coming back to the colours - doctors and nurses - it's a most amazing thing".
The government said testing of NHS staff began over the weekend, with roll-out across England from today.
It was confirmed on Sunday that another 207 people in the UK have died after testing positive for coronavirus - taking the total to 1,235.
Dr Amged El-Hawrani became the first frontline hospital worker in the UK to die after contracting coronavirus.