Staying at home this weekend is an instruction and "not a request", Health Secretary Matt Hancock has said, as he updated the country on the coronavirus.
Speaking at the No 10 briefing, Mr Hancock said that while warm weather was forecast in some areas this weekend "the disease is still spreading".
England's chief nursing officer Ruth May also paid tribute to two nurses who have died from the disease.
"Please stay at home for them," she urged people.
"They were one of us, they were one of my profession, of the NHS family," said Ms May. "I worry that there's going to be more and I want to honour them today and recognise their service."
It comes as latest figures showed a further 684 people with the virus died in the UK, bringing the total to 3,605. There are 38,168 confirmed cases.
In Scotland, the number of deaths has risen by 46, while in Wales a further 24 people died. In NI, the number of people who died with coronavirus has risen by 12.
Mr Hancock - who recently ended his seven days of self-isolation after contracting the virus - said: "We cannot relax our discipline now. If we do, people will die.
"I end with the advice we all know. This advice is not a request - it is an instruction.
"Stay at home, protect lives and then you will be doing your part."
The warning follows messages from local councils, tourism bosses and police urging people to stay away from beauty spots, as the Easter holidays begin and warm weather is expected.
Mr Hancock also said the UK has set up three national clinical trials looking at how existing drugs can be altered to treat Covid-19.
He added that more patients are needed to volunteer to take part in the trials - but England's deputy chief medical officer later clarified that people cannot apply themselves and it was up to doctors to refer patients.
'Stay at home for nurses'
Ms May also appealed to the public to stay at home, saying: "This weekend is going to be very warm and it will be very tempting to go out and enjoy those summer rays.
"But please, I ask to remember Aimee and Areema. Please stay at home for them."
By BBC News health correspondent Lauren Moss
This was one of the longest daily briefings that has been held since the coronavirus outbreak began.
It is the first time we've been given detail of clinical trials which are under way in the UK to treat patients with Covid-19 and find out which medicines will help those who are sick.
At the moment there is no single proven drug to tackle coronavirus and a vaccine is still a long way off.
The trials involve patients from primary to critical care and more than 900 people are already involved.
A word of caution though, even if clinicians give their patients the go ahead to take part, it will probably take a few months for that data to be gathered and then made available.
Also mentioned in Friday's daily update:
- Asked whether face masks were effective, England's deputy chief medical officer Prof Jonathan Van-Tam said "there is no evidence that general wearing of face masks by the public who are well affects the spread of the disease in our society"
- Prof Van-Tam said the government's advisory committee looked into whether the loss of taste and smell is a symptom, as some researchers have suggested. He said while there is anecdotal data some people do lose those senses, the data does not show it is important enough on its own to be added to the list of symptoms
- Discussions over "do not resuscitate" orders should be handled "sensitively", Ms May said - adding these decisions are common but "Covid-19 is no excuse to have those discussions in an insensitive way"
- More than 7,000 NHS staff have now been tested, and more than 2,000 critical care beds - in addition to the Nightingale hospitals - are available
New cases have been slowing down recently: dipping slightly at the weekend and growing more slowly this week (doubling roughly every five days). Even that trend would have predicted over 5,000 new cases today, and so this looks like further evidence that the case numbers could be slowing down (as long as every patient who needs testing is getting tested).
Today's figures on deaths follow the recent trends closely (doubling roughly every 3.5 days).
Remember that doubling every few days means that we should expect to see record new highs regularly.
It takes more than three weeks from infection to death to being reported in these figures.
So while we can hope to see the effects of pre-lockdown social distancing soon, it will take longer for the effect of the lockdown, announced on 23 March, to become apparent.
Meanwhile, the Queen, 93, will speak to the nation on Sunday evening about the coronavirus outbreak in a rare special address.
Buckingham Palace said the message, recorded at Windsor Castle, will be broadcast on TV and radio at 20:00 BST.
The Queen's address will be broadcast on TV, radio and social media, Buckingham Palace said. She has been staying at Windsor Castle since mid-March as a precaution.
It is only her fourth special address at a time of national crisis during her 68-year reign. The other occasions were after the Queen Mother's death in 2002, ahead of Diana, Princess of Wales's funeral in 1997, and during the First Gulf War in 1991.
The Queen also made a televised address to mark her Diamond Jubilee in 2012.
The monarch, 93, released a statement about the outbreak last month, when the number of UK deaths stood at 144.
She said the UK was "entering a period of great concern and uncertainty" and praised the work of scientists, medics and emergency staff, saying everyone has a "vitally important part to play".
BBC royal correspondent Nicholas Witchell said Sunday's speech had been decided "in close consultation with Downing Street" as "they have had it in their minds for some days now".
He suggested the speech might include thanks for NHS staff and key workers, as well as an emphasis on the important role individuals can play - while also aiming to reassure and rally people.
The heir to the throne, Prince Charles, was seen in public for the first time on Friday after being diagnosed with coronavirus and spending seven days in self-isolation.
He opened the first of the National Health Service's emergency field hospitals to treat coronavirus patients in east London's ExCel centre, via a video-link from his home on the Queen's Balmoral estate.
Meanwhile, Prime Minister Boris Johnson, who announced he had contracted the virus last Friday, said he will carry on self-isolating after continuing to display mild symptoms of the virus including a high temperature.