Boris Johnson caught coronavirus after he carried on with normal business and failed to "practise what he preached", scientists have said.
Mr Johnson said today that he had tested positive for coronavirus. He added that he has mild symptoms and is working in isolation.
The Prime Minister has been doing much of his work while social distancing recently, in line with Government rules - including the daily press conference on coronavirus.
But he led Prime Minister's Questions in the House of Commons on Wednesday, which was still attended by dozens of MPs.
Health Secretary Matt Hancock also said he had tested positive on Friday and had mild symptoms.
Last week Mr Hancock met the family of Harry Dunn, a young man killed by a car driving on the wrong side of the road, to discuss ambulance response times.
A spokesperson for the Dunn family said that Mr Hancock had hugged and shaken hands with the family - going against the Government's social distancing rules.
Professor Susan Michie, a behavioural psychologist at University College London, said it was unsurprising Mr Johnson and Mr Hancock had caught coronavirus, given that the virus spreads through social contact.
Ms Michie said: "Whilst the PM was telling people to stay at home and keep at least two metres apart from each other, the House of Commons was open for business and face-to-face parliamentary activities were carrying on.
“Given the transmission routes of touching contaminated surfaces and breathing in virus-laden droplets, it should not come as a surprise to hear that the PM and Health Secretary have tested positive for coronavirus."
She added that there are several reasons why leaders "should practise what they preach".
Ms Michie said: “The first is that such people are important role models, with the ability to enhance or undermine their verbal messages by their actions.
“The second concerns trust: if leaders do not adhere to their own recommendations, this undermines trust in them which in turn can undermine the population’s adherence to their advice.”
Others are concerned that Mr Johnson's diagnosis could distract from the wider pandemic.
Devi Sridhar, professor of global public health at the University of Edinburgh, said: “I fear the PM news today will swamp headlines and distract from core issues.
“We need governments to present clear strategy with deadlines attached for next few weeks.
"What is the concrete plan for testing? What is the plan for personal protective equipment for health workers?
Downing Street said on Thursday that the Government had ordered 8,000 additional ventilators on top of the 8,000 already available.
Officials said thousands more would be available in the “coming weeks”, with the peak of the virus expected to hit within the next fortnight.
Mr Johnson took part in a conference call with manufacturers on Friday to discuss the progress of ventilator orders.
Fellow minister Michael Gove defended his Government colleagues on Friday in a press conference .
Mr Gove said: “The fact that the virus is no respecter of individuals, whoever they are, is one of the reasons why we do need to have strict social distancing measures so that we can reduce the rate of infection and reduce the pressure on the NHS.”
NHS England suggests that people who have had in-person meetings with Mr Johnson or Mr Hancock but do not live with them do not need to self-isolate unless they show symptoms themselves.
A spokesman told the Standard: "Everyone is being told to stay at home unless there's a good reason for them not to do so."
The UK's chief medical officer Chris Whitty is also in isolation after feeling coronavirus symptoms, he said on Friday.
A total of 14,579 people have tested positive for coronavirus in the UK as of Thursday evening, while 759 have died after being diagnosed with the virus.