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 receive a leaflet detailing the government’s coronavirus rules and health information.
It follows criticism over the clarity of government advice to date.
The number of people to have died with coronavirus in the UK has now reached 1,019.
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.”
The prime minister will tell the nation: “It’s important for me to level with you – we know things will get worse before they get better.
“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.”
Mr Johnson will describe the pandemic as a “moment of national emergency” , urging the public to stay at home, protect the NHS and save lives.
He will also praise 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 will include guidance on handwashing, a clear 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 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.