Boris Johnson is writing to every household in the UK to urge the public to obey the lockdown and stay home during the coronavirus "national emergency".
The Prime Minister, who is self-isolating after testing positive for Covid-19, will warn "things will get worse before they get better" as he stresses the need to stay indoors to support the NHS by slowing the spread.
At an anticipated cost of £5.8 million, the letters will land on 30 million doorsteps along with a leaflet spelling out the Government's advice following much public confusion.
They are the latest in a public information campaign from No 10 to convince people to stay at home, wash their hands and shield the most vulnerable from the disease.
"We know things will get worse before they get better," the PM's letter will read.
"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.
"It has been truly inspirational to see our doctors, nurses and other carers rise magnificently to the needs of the hour.
"Thousands of retired doctors and nurses are returning to the NHS – and hundreds of thousands of citizens are volunteering to help the most vulnerable.
"That is why, at this moment of national emergency, I urge you, please, to stay at home, protect the NHS and save lives."
Amid allegations of confusing messages on the lockdown, the leaflet will outline the Government's rules on leaving the house and advice on shielding vulnerable people.
A clear explanation of the symptoms will also be included as will guidance on hand washing.