In a televised address to the nation at 7:00 pm (1800 GMT), Johnson is expected to extend confinement orders imposed in late March, although garden centres are set to reopen.
But in a sign of a gradual shift towards getting the country moving again, he tweeted new public advice for people to "stay alert", to replace the current slogan to "stay home, save lives".
People must stay at home "as much as possible" and limit contact with others, a more lenient message than previous rules to only go outside when necessary and to meet nobody outside your of own household.
Johnson, who spent a week in hospital with coronavirus last month, has said he will proceed with "maximum caution" in lifting the lockdown.
He is expected to extend the rules on Sunday by announcing a mandatory 14-day quarantine for anyone arriving into the UK.
However, his new 'stay alert' slogan drew criticism from Scottish First Minister Nicola Sturgeon, who warned infection rate in her country remained too high to ease public advice.
"We mustn't squander progress by easing up too soon, or by sending mixed messages that result in people thinking that it's OK to ease up now," she told reporters.
"Let me be very blunt about the consequences if we were to do that -- people will die unnecessarily."
The devolved governments in Scotland, Wales and Northern Ireland have already extended their lockdowns, and Johnson's plan will focus on England.
He will also unveil an alert system to monitor the outbreak, which will inform the use of lockdown measures at a national and local level.
- Don't run too fast -
Johnson has been criticised for failing to take the outbreak seriously enough at the start, still shaking hands with people in early March and delaying the imposition of a lockdown.
Britain has now recorded 31,855 deaths among people who have tested positive for COVID-19 -- an increase of 269 on the day before, and the second highest figure in the world after the United States.
There are growing demands from his own MPs to get the economy moving again, particularly after the Bank of England predicted a 14-percent slump in British GDP this year.
But in an interview with the Sun on Sunday newspaper, Johnson warned that now was "the most dangerous bit".
"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," he said.
"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."
- Remain vigilant -
The government has had to lower expectations about Johnson's address, after newspaper reports earlier this week suggested the lockdown would be eased.
The vast majority of people have kept to the rules but crowds flocked to parks this weekend to take advantage of the hot weather, and there are concerns the new "stay alert" slogan could encourage more to venture out.
Communities Secretary Robert Jenrick said the updated message was about people taking "personal responsibility" by maintaining social distancing and washing their hands.
"We should be staying at home as much as possible but when we do go to work and go back to our business we need to remain vigilant," he told Sky News television.
The new alert system, led by a new centre for biosecurity, will assess the risk of coronavirus at one of five levels and monitor the impact of any changes to the lockdown.
Britain is also trialling a new phone app to identify localised outbreaks, and in recent weeks has increased its capacity to test for coronavirus to around 100,000 a day.
This story has been published from a wire agency feed without modifications to the text. Only the headline has been changed.