The Prime Minister, 55, will reportedly urge “maximum caution” when he unveils his road map out of the lockdown.
The risks now are said to be greater than at any stage of the crisis, which led to the stay-at-home restrictions being imposed on March 23.
But, during the address, Mr Johnson will admit the country is past the peak of the infection, reports suggest tonight.
“This is a critical moment,” a Downing Street source told The Sun on Sunday.
The Prime Minister told the publication the country won’t rush out of the lockdown.
“This is the dangerous bit,” he said.
“We’re past the peak now but we’ll have to work even harder to get every step right.
“Mountaineers always say that coming down from the peak is the most dangerous bit. That’s when you’re liable to be over-confident and make mistakes.
“You have very few options on the climb up — but it’s on the descent you have to make sure you don’t run too fast, lose control and stumble.”
Millions are expected to watch Mr Johnson’s speech, in which he reportedly will state the new catchphrase replacing the “stay at home” mantra.
But the UK death toll today exceeded 31,500 in total, and a further 215,200 have been infected by Covid-19 in the country.
Garden centres will be allowed to reopen next week.
Mr Johnson will also tomorrow give guidance to employers so they can ensure a safe and strictly-controlled working environment for returning staff.
The PM will chair a meeting of the emergency Cobra committee with Cabinet ministers, leaders of the devolved nations and London Mayor Sadiq Khan before his 7pm pre-recorded address.
On Monday, the Government will publish a 50-page document outlining the full plan to cautiously re-start the economy to MPs.
The shift in messaging will come amid concerns that workers may not wish to resume their roles after the firm instructions to “stay at home, protect the NHS, save lives”.
Later in the week, Mr Johnson will address the 1922 Committee of backbench Conservatives.