Boris Johnson is warning every household he could impose even stricter lockdown measures to tackle the coronavirus outbreak.
The Prime Minister - who is self-isolating with Covid-19 - is writing to every address in the UK telling people the closer they adhere to the rules "the sooner life can return to normal".
The letters will land on doorsteps across the country, stressing outbreak is a "national emergency".
It comes as the number of people to have died in UK hospitals after contracting the virus surged past 1,000 - increasing by 260 in just 24 hours.
Elsehwere NHS England's national medical director warned that now was not the time for complacency after a study suggested social distancing could deliver a lower death toll than previously feared.
While Scottish Secretary Alister Jack became the latest Cabinet minister to enter self-isolation with Covid-19 symptoms.
Both Boris Johnson and Health Secretary Matt Hancock confirmed they had tested positive for the virus on Friday.
In letters to 30 million households, Mr Johnson writes: "We will not hesitate to go further if that is what the scientific and medical advice tells us we must do.
"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 last week offered a glimmer of hope when he said he expects the UK can "turn the tide" within three months.
A lengthier estimate for the lockdown was offered, however, by a scientist whose research has been key in the Government's approach.
Imperial College London's Professor Neil Ferguson told the Sunday Times: "We're going to have to keep these measures [the full lockdown] in place, in my view, for a significant period of time".
He estimated: "Probably until the end of May, maybe even early June."
NHS England's national medical director Professor Stephen Powis appeared at the daily press conference as Mr Johnson worked behind closed doors in his Downing Street flat.
Prof Powis warned the public not to rest on their laurels after an Imperial College London study suggested the UK could be on course for 5,700 deaths if it follows the same trajectory as China.
He said it would be a "good result" if the toll in the UK was less than 20,000, and stressed compliance with the strict rule - and not luck - would get the number down.
Prof Powis added: "But we shouldn't be complacent about that, although that would be a good result, it will only happen if we stop the transmission of the virus".
Elsewhere work has been rapidly going ahead converting London's ExCel convention centre into a field hospital dubbed the NHS Nightingale Hospital.
While coronavirus tests are being trialled for NHS staff so frontline workers self-isolating with potential symptoms may be able to get the all clear and return to work.
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