Prime Minister Boris Johnson has urged caution as most legal restrictions on social contact are lifted in England.
There are now no limits on how many people can meet or attend events; nightclubs reopened at midnight; and table service will not be necessary in pubs and restaurants.
Face coverings will be recommended in some spaces, but not required by law.
Vaccines Minister Nadhim Zahawi said government experts had recommended that only some under-18s be vaccinated.
The prime minister is due to hold a news conference at 17:00 BST via video link as he, the chancellor and the health secretary are self-isolating – amid warnings that cases of Covid will surge.
On BBC Radio 4’s Today programme, Mr Zahawi defended the U-turn which saw the prime minister and chancellor self-isolating, after initially saying they would take part in a pilot programme to take daily tests instead.
Mr Zahawi insisted that Mr Johnson only “considered” joining the pilot scheme, adding: “No one is taking anyone for fools.”
Labour leader Sir Keir Starmer accused the government of “chaos, confusion and cronyism”, with the prime minister and chancellor undermining the self-isolation rules at a critical moment.
He said a “reckless free-for-all” now risked plunging the country back into restrictions. “The government has once again lost control of the virus. They need to think again on their reckless plan before it’s too late,” he said.
Some scientists are predicting that UK infections – currently at about 50,000 a day – could reach 200,000 a day later in the summer.
But with more than 68% of UK adults fully vaccinated, modelling suggests hospital admissions, serious illness and deaths from Covid-19 will be at a lower level than in earlier peaks.
In a video posted on Twitter on Sunday afternoon, the prime minister said now was the “right moment” to move to the final stage of England’s roadmap out of lockdown.
“If we don’t do it now we’ve got to ask ourselves, when will we ever do it?” he said, adding that the virus would have the “advantage of the cold weather” in autumn and winter.
“But we’ve got to do it cautiously. We’ve got to remember that this virus is sadly still out there. Cases are rising, we can see the extreme contagiousness of the Delta variant.”
Nadhim Zahawi said 90% of the most vulnerable had been vaccinated and it was right for people to take “personal and corporate responsibility” over measures such as wearing masks.
“We’re doing the right thing to get as close to normal as possible, as quickly as possible,” he told BBC Breakfast, saying that the end of the school term would reduce the spread of the virus.
He also said he had received the recommendation from the Joint Committee on Vaccination and Immunisation about vaccinating children aged 12 to 17 and would make a statement to Parliament on Monday afternoon.