Boris Johnson is to urge the public to "act responsibly" when lockdown is eased in England on Saturday.
A number of businesses - including restaurants, pubs, hairdressers and cinemas - will be allowed to open their doors for the first time since March.
But at a press conference on Friday, the PM will warn "we are not out of the woods yet" when it comes to the virus.
The rule change comes days after a local lockdown was put in place in Leicester because of a spike in cases.
Businesses and schools in the city will have to remain closed, with residents advised not to travel.
Mr Johnson announced the upcoming changes to lockdown measures last week, along with an easing of social distancing rules from 2m to "one metre plus".
In Friday's speech, he is expected to say the move is about "supporting the livelihoods of business owners and their employees up and down the country".
He will praise the "heroic efforts" of the businesses to get ready for Saturday, but will add: "The success of these businesses, the livelihoods of those who rely on them, and ultimately the economic health of the whole country is dependent on every single one of us acting responsibly. We must not let them down.
"Just as when we first locked down, we will only succeed in reopening if everyone works together. Because we are not out of the woods yet. The virus is still with us and the spike in Leicester has shown that."/GETTY
The PM will also reiterate that the government "will not hesitate in putting on the brakes and re-imposing restrictions" if there is a spike in the virus as a result of the changes.
"Anyone who flouts social distancing and Covid-secure rules is not only putting us all at risk but letting down those businesses and workers who have done so much to prepare for this new normal," he will add.
Each UK nation's lockdown measures differ, including varying rules on the reopening of food and drink outlets.
In Scotland, beer gardens and outdoor restaurants will be allowed to reopen from 6 July, and indoor areas can be used from 15 July. In Northern Ireland, pubs and restaurants can open from 3 July.
The Welsh government has promised talks with the hospitality sector about a "potential phased" reopening, but no dates have yet been given.