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 PM, chancellor and the health secretary are self-isolating, and there are warnings 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.”
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.”
Labour’s shadow health secretary Jonathan Ashworth said ending legal mask requirements and the recommendation to work from home was “reckless”, and warned of a “day of chaos” on the transport network as people returned to offices after months away.
“The last thing we want on a day like this is chaos and for the public health measures to be undermined,” he told BBC Radio 4’s Today programme.