Coronavirus curbs were eased in England, Wales and most of Scotland.
Millions of people can now socialise indoors in limited numbers, hug loved ones and visit pubs, bars and restaurants inside again.
Entertainment venues such as cinemas, theatres, concert halls, museums and sports stadiums can all reopen.
The ban on foreign travel has also been lifted and replaced with new rules.
Boris Johnson said everyone still needed to be cautious, and also get tested twice a week.
“Together we have reached another milestone in our roadmap out of lockdown, but we must take this next step with a heavy dose of caution,” said the prime minister.
He added: “Everyone must play their part by getting tested twice a week, coming forward for your vaccine when called and remembering hands, face, space and fresh air.
“I urge everyone to be cautious and take responsibility when enjoying new freedoms today in order to keep the virus at bay.”
Anyone in England and Scotland can order free lateral flow tests, which give results in 30 minutes even if they don’t have symptoms. In Wales and Northern Ireland, they are available for certain people, such as those who cannot work from home.
Restrictions however remain in Scotland’s Glasgow and Moray after a recent spike in cases.
The changes came even as the Indian variant continues to spread in the UK, with mass testing rolled out to hotspots such as Bolton in Greater Manchester and parts of London, Sefton and Worcestershire.
Over the weekend the British Medical Association said it was a “real worry” that the easing was still going ahead while the Indian variant was spreading and many younger people were not vaccinated.
Mr Johnson said the government was keeping the variant “under close observation” and “taking swift action where infection rates are rising”.
On Sunday government figures showed a further 1,926 Covid cases, while a further four people have died.