The prime minister has warned the coronavirus crisis will get worse before it gets better, in a letter being sent to every UK household.
Boris Johnson, who is self-isolating after testing positive for Covid-19, says stricter restrictions could be put in place if necessary.
Britons will also get a leaflet detailing government rules on leaving the house and health information.
It follows criticism over the clarity of government advice to date.
The number of people who have died with coronavirus in the UK has now reached 1,019, with a further 260 deaths announced on Saturday.
There are now 17,089 confirmed cases in the UK.
In the letter being sent to 30 million households at an anticipated cost of £5.8m, Mr Johnson writes: "From the start, we have sought to put in the right measures at the right time.
"We will not hesitate to go further if that is what the scientific and medical advice tells us we must do."
Tough measures to tackle the spread of coronavirus, including a ban on public gatherings of more than two people and the closure of shops selling non-essential goods, were introduced last week.
"It's important for me to level with you - we know things will get worse before they get better," the letter reads.
"But we are making the right preparations, and the more we all follow the rules, the fewer lives will be lost and the sooner life can return to normal."
Experts have said they expect the number of coronavirus cases and deaths to continue to rise for the next two to three weeks, before the effects of social distancing measures and restrictions on everyday life begin to have an impact.
In his letter, Mr Johnson describes the pandemic as a "moment of national emergency", and reiterates the government guidance to stay at home to avoid putting pressure on the NHS, so that lives can be saved.
The prime minister acknowledges the restrictions will have a financial impact on families.
However, writing after ministers unveiled billions of pounds of help for workers and business, he says: "The government will do whatever it takes to help you make ends meet and put food on the table."
<blockquote class="twitter-tweet"><p lang="en" dir="ltr">PM <a href="https://twitter.com/BorisJohnson?ref_src=twsrc%5Etfw">@BorisJohnson</a> is writing to every UK household to urge them to stay at home, protect the NHS and save lives. <a href="https://twitter.com/hashtag/StayHomeSaveLives?src=hash&ref_src=twsrc%5Etfw">#StayHomeSaveLives</a> <a href="https://t.co/GMNPqEl10d">pic.twitter.com/GMNPqEl10d</a></p>— UK Prime Minister (@10DowningStreet) <a href="https://twitter.com/10DowningStreet/status/1244029214933889024?ref_src=twsrc%5Etfw">March 28, 2020</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>
Mr Johnson also praises the work of doctors, nurses and other carers as well as well as the hundreds of thousands of people who have volunteered to help the most vulnerable.
The leaflet sent alongside the letter includes guidance on handwashing, an explanation of coronavirus symptoms, the government rules on leaving the house and advice on shielding vulnerable people.
Meanwhile, new powers, including fines of up to £5,000, to enforce guidelines on people staying at home and businesses staying closed came into force in Northern Ireland on Saturday evening.
The maximum fine will be reserved for businesses but individuals could face a fine of up to £960 if they do not comply.
In other developments:
The first of three flights chartered by the Foreign Office to bring home British tourists stranded in Peru will leave the capital Lima on Sunday
The Duke and Duchess of Cambridge have lent their support to a Public Health England initiative giving people tips on how to look after their mental health during the pandemic as the government provides an additional £5m funding to charities to expand their services
More than 750 St John Ambulance volunteers have offered to help at the temporary hospital set up in London's Excel centre to meet demand for intensive care beds
Food parcels have begun to be delivered to the most vulnerable people in England who are being told to stay at home for the next three months to protect them from coronavirus
Secretary of State for Scotland, Alister Jack, is self-isolating after he developed coronavirus symptoms. He is said to be experiencing mild symptoms but has not been tested for Covid-19
Writing in the Mail on Sunday, Home Secretary Priti Patel has pledged to protect victims of domestic abuse, who she says are particularly at risk during the pandemic because of the need to stay at home
The Mayor of Greater Manchester, Andy Burnham, said he had received about 300 complaints from employees and the public about businesses not following social distancing guidelines
The Foreign Office said it was working "around the clock" to support British travellers stranded in India amid a government lockdown
The prime minister's letter follows comments by the medical director of NHS England, who said on Saturday "now is not the time to be complacent" over the virus.
Speaking at the government's daily coronavirus briefing, Prof Stephen Powis said "every one of us" had a part to play if the UK was to keep the death toll under 20,000.
Business Secretary Alok Sharma also announced insolvency rules would be changed to allow firms greater flexibility as they faced the coronavirus crisis.
He said a range of measures to boost the supply of personal protective equipment, such as face masks, to protect frontline NHS staff, were also being introduced.
It comes as testing begins to be rolled out for frontline hospital staff in England to see whether they have coronavirus.
In Wales and Scotland, health workers are already being tested. Northern Ireland's testing begins on Monday.
As well as the prime minister, Health Secretary Matt Hancock is also self-isolating after testing positive for the virus. The UK's chief medical officer, Prof Chris Whitty, is also self-isolating, but has not tested positive.
https://www.bbc.com/news/uk-politics-52079922
