Video report by ITV News Political Correspondent Romilly Weeks
On his return to work following his treatment for Covid-19, a virus he described as an "invisible mugger", the prime minister said social distancing was working, but said now is the "moment of maximum risk".
Speaking from outside Downing Street, where he has not been seen in public since being taken to hospital on April 5, Mr Johnson said the UK is "coming to the end of the first phase of the conflict".
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He encouraged people to "contain your impatience" with lockdown, by sticking with restrictions, but his speech was short of detail explaining just how measures may eventually be eased.
He's been facing pressure from within his own party and from Labour leader Sir Keir Starmer, to release his lockdown "exit strategy" but he resisted temptation, saying there'll be "much more" detail in coming days.
He said once the government's "five tests" had been met, then restrictions would "gradually" ease, in order to "one-by-one to fire up the engines of this vast UK economy".
But he added: "We simply cannot spell out now how fast or slow or even when those changes will be made."
He said there is a danger that people would look at the “apparent success” and “go easy” on social distancing measures.
His claim that the UK had handled the pandemic successfully was questioned on social media, with many pointing to Britain's high position in global death tolls.
The UK is just one of a handful of countries to have a coronavirus-related deaths over 20,000, but the true total is believed to be significantly higher than than.
As of Sunday, the UK had recorded 20,732 Covid-19 fatalities, however that figure only takes in deaths which have occurred in hospitals and it is thought thousands more will have died in settings in the community, such as care homes.
Mr Johnson acknowledged how debilitating the virus can be, drawing on his own experience, he described it as a "a physical assailant, an unexpected and invisible mugger".
He admitted he'd been “away from my desk for much longer than I would’ve liked”, after returning to work following a stay in intensive care in which the NHS "saved" his life.
But the prime minister appeared to be back to his bullish self while giving his speech on the steps of Downing Street.
He was animated, gesturing at the lectern, with Health Secretary Matt Hancock saying he was on "top form".
He thanked those who had “stepped up” in his absence, including his stand-in Dominic Raab.
Mr Johnson prepared for his return with a three-hour summit meeting on Friday with Mr Raab and Chancellor Rishi Sunak.
The prime minister said the UK "collectively shielded our NHS" so doctors and nurses could "shield all of us from an outbreak".
It remains the “biggest single challenge this country has faced since the war”, Mr Johnson said, and “every day I know that this virus brings new sadness and mourning to households across the land”.
Mr Johnson returned back to Number 10 on Sunday evening, around a month since he was diagnosed with coronavirus.
He spent a week at St Thomas' Hospital in central London, including three nights in intensive care.
He will chair the morning Cabinet meeting on Covid-19 and hold talks with senior officials and ministers.
Coronavirus: Everything you need to know