Restrictions banning social gatherings of more than six people have come into effect, following a rise in coronavirus cases.
The “rule of six” applies in both indoors and outdoors in England and Scotland, and indoors only in Wales.
Restrictions in England affect everyone, but do not include children under 11 in Wales or under 12 in Scotland.
Some Tory MPs have urged No 10 to also exempt young children in England.
The new rules are introduced as the UK’s R number escalates to between 1 and 1.2 for the first time since March, indicating infections are increasing.
A further 3,330 positive cases were recorded in the UK on Sunday – the third consecutive day in which cases have been over 3,000 – with five more deaths reported.
Home Secretary Priti Patel urged the public not to break the rules, as police forces vowed to enforce them.
Cabinet Office minister Michael Gove defended the new measures in England as “absolutely right”, saying some people had “unwittingly” contributed to the spread of the virus because of the way they had interacted.
Under the new rules, larger gatherings are still allowed at gyms, schools, places of worship, weddings and funerals. Education and work settings are not affected.
The “rule of six” in England replaces the existing ban on gatherings of more than 30 people and the current guidance on allowing two households to meet indoors.
England, Scotland and Wales each have new restrictions.
Groups larger than six can be broken up by police, with members of the group facing fines of £100 for a first offence, doubling on each further offence up to £3,200.
Northern Ireland announced a six person, two household limit on indoor gatherings on 24 August, a reduction on a previous restriction of 10 people from four households.