Two households or groups of up to six people are now able to meet outside in England again as the stay-at-home Covid restrictions order comes to an end.
Outdoor sport facilities including tennis courts and golf courses are also reopening, and organised outdoor sports allowed to resume in the latest easing.
And weddings will also be on again, attended by up to six people.
But Boris Johnson urged caution, saying cases were up in Europe and variants threaten the vaccine rollout.
The prime minister said: “Despite today’s easements, everyone must continue to stick to the rules, remember hands, face, space, and come forward for a vaccine when called.”
A new slogan – Hands, Face, Space and Fresh Air – was also being unveiled by the government to emphasise the importance of ventilation in reducing the spread of the virus.
Prof Chris Whitty, the UK’s chief medical officer, said: “The evidence is very clear that outdoor spaces are safer than indoors. It is important to remember this as we move into the next phase.”
Monday marks the second easing of the lockdown imposed across England in early January, after schools reopened to all pupils on 8 March. People were also permitted to exercise with their household, or with one person from outside their household.
People will be allowed to meet outside – including in private gardens – in groups of up to six, or as two households, with social distancing.
But as many friends and families prepare to be reunited for the first time together in several months, ministers are still advising people to work from home where possible and minimise the number of journeys they take.
The next stage in the relaxation of the rules will take place no earlier than 12 April with non-essential retail reopening, and restaurants and pubs allowed to serve outdoors.
In Wales, the “stay local” rule was lifted on Saturday, with six people allowed to meet outside and stay in self-contained holiday accommodation.
The stay home order in Scotland will end on Friday, and from Thursday in Northern Ireland up to six people, or two households, will be able to meet outdoors.
Another 19 deaths of people within 28 days of a positive Covid test were recorded on Sunday, compared to 33 on the same day last week, along with a further 3,862 cases.
The latest government figures show 423,852 UK adults received a first dose of the vaccine on Saturday, taking the overall number to 30,151,287 – with 233,964 having their second dose, bringing that total to 3,527,481.
Sports stars including England World Cup-winning cricket captain Eoin Morgan and British number one women’s tennis player Johanna Konta are teaming up with the government to encourage a return to physical activity.
Mr Johnson said “as teams return to outdoor pitches, courts, parks and fields, I hope today will kick-start a Great British summer of sport”.
The prime minister added: “I know how much people have missed the camaraderie and competition of organised sport, and how difficult it has been to restrict physical activities – especially for children.”
The government has announced it will set up a new Office for Health Promotion in England to promote on physical fitness and mental health, saying the pandemic has revealed many vulnerabilities in the population.
Meanwhile, English Heritage is scheduled to reopen more than 50 outdoor sites on Monday that have been closed to the public since December.