The PM has said that since the UK has "past the peak" of the coronavirus outbreak and is presently "on the downward slope" he will set out a "comprehensive" plan next week on reopening schools and restarting the economy.
There have been 26,711 deaths in UK hospitals and the wider community, Mr Johnson said.
The prime minister insisted that to avoid the "disaster" of a second peak, despite the fact that "we can now see the sunlight", the UK must meet the fifth of five tests before the lockdown can be lifted.
"Nothing we do should lift the R or reproduction rate - back above one," he said.
Later in the briefing, the BBC's political editor Laura Kuenssberg asked what level the reproduction rate should be before the government would be "comfortable easing restrictions".
The government's chief medical officer, Prof Chris Whitty, replied: "We are absolutely confident that the wrong answer is anything over one."
He explained that as soon as the R rises above one you "restart exponential growth" and "sooner or later" the NHS would be at the risk of being overwhelmed.
Mr Johnson said that keeping the reproduction rate down "is going to be absolutely vital to our recovery".
The UK government has said that these five tests have to be met before they will consider easing coronavirus lockdown restrictions:
The NHS has sufficient capacity to provide critical care and specialist treatment right across the UK
A sustained and consistent fall in daily deaths from Coronavirus
Reliable data to show that the rate of infection is decreasing to manageable levels across the board
Operational challenges including testing and personal protective equipement (PPE) are in hand with supply able to meet future demand
Confident that any adjustments to the current measures will not risk a second peak of infections that overwhelms the NHS