Prime Minister Boris Johnson has tested positive for coronavirus and is self-isolating in Downing Street.
He said he had experienced mild symptoms over the past 24 hours, including a temperature and cough, but would continue to lead the government.
England's Health Secretary Matt Hancock said he had also tested positive while England's Chief Medical Officer, Prof Chris Whitty, has shown symptoms.
Another 181 people died with the virus in the past day, figures showed.
It takes the total number of UK deaths to 759, with 14,543 confirmed cases.
The daily coronavirus news conference was led by Cabinet Office minister Michael Gove, alongside deputy chief medical officer Dr Jenny Harries and NHS England chief executive Sir Simon Stevens.
They announced plans to begin a large-scale testing programme of health service staff, starting with critical care teams. It will later be expanded to cover social care staff too.
It follows mounting criticism from NHS staff over a lack of testing - currently, only seriously-ill patients in hospital are being tested.
Testing will be carried out on staff showing possible symptoms of the virus or staff who live with people who have symptoms - not for all frontline workers as a matter of course.
"This will be antigen testing - testing whether people currently have the disease - so that our health and social care workers can have security in the knowledge that they are safe to return to work if their test is negative," Mr Gove said.
The British Medical Association, which represents doctors, said the announcement was "long overdue" and the lack of testing so far had been "incredibly frustrating".
"For every healthy member of staff at home self-isolating needlessly when they do not have the virus, the NHS is short of someone who could be providing vital care to patients on the frontline," BMA chairman Dr Chaand Nagpaul said.
He said that 33,000 beds - the equivalent of 50 hospitals - had been freed up across England ready for coronavirus patients.
The government has imposed strict restrictions on everyday life designed to slow the spread of the virus.
However, BBC medical correspondent Fergus Walsh said the UK's daily death toll will still rise into the many hundreds in the coming weeks.
This does not mean social distancing measures are not working - but there will be a lag of two to four weeks before we see the effects, our correspondent says.
Mr Gove said scientific analysis suggested the rate of infection had been doubling every three to four days, but the "fantastic" public response to the restrictions would make a difference.
Asked whether the prime minister and health secretary should have been "better protected", he 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," he added.
Working from home
Mr Johnson is thought to be the first world leader to announce they have the virus.
He was last seen on Thursday night, clapping outside No 10 as part of a nationwide gesture to thank NHS staff and carers.
Over the last 24 hours I have developed mild symptoms and tested positive for coronavirus.— Boris Johnson #StayHomeSaveLives (@BorisJohnson) March 27, 2020
I am now self-isolating, but I will continue to lead the government’s response via video-conference as we fight this virus.
Together we will beat this. #StayHomeSaveLives pic.twitter.com/9Te6aFP0Ri
In a video on his Twitter account, Mr Johnson, 55, said: "I'm working from home and self-isolating and that's entirely the right thing to do.
"But, be in no doubt that I can continue, thanks to the wizardry of modern technology, to communicate with all my top team to lead the national fight-back against coronavirus."
The PM chaired a phone call on Friday morning, and later in the day, Downing Street said he had spoken to US President Donald Trump.
"The president wished the prime minister a speedy recovery from coronavirus," a spokesman said. "They agreed to work together closely, along with the G7, the G20, and other international partners, to defeat the coronavirus pandemic."
Mr Hancock said he was experiencing mild symptoms of the virus, and would be self-isolating until next Thursday.
He told BBC Look East it was "understandable that people will ask the question" why he and the prime minister were tested, but most people with possible symptoms were not.
The health secretary said there was a protocol laid down by the chief medical officer which required a small number of senior figures, key to the national effort, to be tested.
Earlier this week the prime minister's spokesman said if Mr Johnson was unwell and unable to work, Foreign Secretary Dominic Raab, as the first secretary of state, would stand in.
The prime minister's fiancée, Carrie Symonds, who is several months pregnant, is also self-isolating, although it is not known if they are still living together.
Pregnant women in their third trimester are advised to be particularly stringent when following social distancing advice, and minimise social contact for up to 12 weeks.
Maybe it was inevitable.
One of the first moments that raised eyebrows in the course of the UK outbreak was when health minister Nadine Dorries came down with coronavirus.
Then, last week, we discovered that some key staff in No 10, including the prime minister's chief Brexit negotiator David Frost, were self-isolating with suspected symptoms.
A fair number of MPs took themselves off into isolation for fear of having contracted the infection.
Their remaining colleagues were continually ordered to sit far apart on the green benches, before finally, this week, Parliament itself closed early, with no certain date for a return of normal business.
Still, the news this morning that the prime minister himself has contracted coronavirus felt like a shock.
Neither the PM's senior adviser Dominic Cummings nor Chancellor Rishi Sunak has symptoms. They have not been tested.
A Buckingham Palace spokesman confirmed the Queen, 93, saw Mr Johnson more than two weeks ago on 11 March, and she is in good health.
The pair usually meet weekly for the prime minister's audience with the Queen, but the most recent meetings have been over the phone.
In other developments:
- Police forces in England and Wales have fined people for ignoring guidance issued to prevent the spread of coronavirus.
- The UK's chief scientific adviser Sir Patrick Vallance has said he expects the number of coronavirus cases to increase over the next two to three weeks, and then start to gradually decrease due to the current lockdown.
- Mayor Sadiq Khan has warned Londoners there will be "a large number" of deaths and temporary mortuaries will be set up across the capital.
- The Ministry of Justice says 27 prisoners, in 14 prisons, have tested positive.
- Italian restaurant chain Carluccio's is facing collapse, after warning it is facing permanent branch closures due to coronavirus.
- The number of people who have volunteered to help the NHS has reached 700,000. The government originally set a target of 250,000 but increased it to 750,000 after a huge response.
- More than 7,000 former nurses and midwives have signed up to return to the profession.
- Tesco has said that online shoppers will have their deliveries capped at 80 items from now on, while supermarkets will also use a government database of 1.5 million vulnerable shoppers in England to help prioritise delivery slots.
- The Queen's birthday parade, the Trooping the Colour, will not go ahead in its usual form, and other options are being considered.
Other world leaders including Canada's Justin Trudeau and Germany's Angela Merkel have self-isolated after coming into contact with people who have tested positive for the virus.
Politicians including Labour's shadow home secretary Diane Abbott shared messages to the PM, wishing him a "speedy recovery".
Meanwhile, the president of the European Council, Charles Michel, told Mr Johnson: "Europe wishes you a speedy recovery."
In the Irish Republic, the Taoiseach, Leo Varadkar, has announced tough new restrictions on movement.
From midnight on Friday, for a two-week period until Easter Sunday, everyone must stay at home unless their work is essential or they are buying food.
Twenty two people with coronavirus have died in the Irish Republic.