Another 209 people have died in the UK after testing positive for coronavirus, health officials have confirmed.
The Department of Health and Social Care said a total of 1,228 patients have now died as of 5pm Saturday, up from 1,019 the day before.
There are now 19,522 cases in the UK, from a total of 127,737 tests.
Health chief Prof Neil Ferguson also cautioned that the social distancing rules may need to last until June.
He told the Sunday Times: “We are going to have to keep these measures in place, in my view, for a significant period of time - probably until the end of May, maybe even early June. May is optimistic."
The jump in fatalities - a slight dip on the daily record of 260 on Saturday - comes at the end of the first week of unprecedented curbs on daily life.
The Government has banned Brits from leaving home for anything but jobs, essentials or to care for a vulnerable person, and all "non-essential" shops are shut nationwide.
Police across the UK have been granted powers to fine and arrest those who refuse to comply, however Home Secretary Priti Patel said on Sunday that domestic abuse victims can still leave the house to seek help at refuges.
In his letter to British households - costing an estimated £5.8m - the Prime Minister describes the pandemic as a "moment of national emergency".
"From the start, we have sought to put in the right measures at the right time," Mr Johnson writes.
"We will not hesitate to go further if that is what the scientific and medical advice tells us we must do."
It will arrive on doormats with a brochure from Public Health England (PHE) issuing fresh guidance on hygiene and shielding vulnerable people.
The letter continues: "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."
Soldiers have been drafted in to build a 4,000-bed field hospital at the ExCel Centre in London's Docklands, set to open next week, and critical care beds have been ramped up in the capital.
It is one of five NHS Nightingale temporary hospitals opening across the UK to cope with a surge in patients over the coming weeks as the virus peaks.
It comes as applications for the NHS 'volunteer army' shut this afternoon as applications smashed the initial target three times over, hitting 750,000.
The Duke and Duchess of Cambridge are also launching a £5m campaign with PHE to protect the mental health of high-risk people told to self-isolate for three months.