The further easing of lockdown restrictions in England – due to come in this weekend – has been postponed for at least two weeks, amid concerns over an increase in coronavirus cases.
Casinos and bowling alleys will remain shut, with Boris Johnson saying it was time to “squeeze the brake pedal”.
Wedding receptions of up to 30 people were meant to be allowed as part of the changes but cannot yet happen.
Face coverings will be mandatory in more indoor settings, such as cinemas.
People attending places of worship will also be required to wear face coverings, in a change that will be applied from next weekend.
But planned changes to guidance for those who have been shielding during the pandemic, and advice for employers, will still go ahead.
Mr Johnson made the comments during a special Downing Street briefing, where he was joined by England’s chief medical officer, Prof Chris Whitty.
Prof Whitty warned that it might not be possible to ease the lockdown any further, explaining that the “idea that we can open up everything and keep the virus under control” is wrong.
Asked whether it was safe for England’s schools to open fully to all pupils in the autumn, he said it was a “difficult balancing act” but “we have probably reached or neared the limits of what we can do in terms of opening up society”.
The rethink follows new restrictions for people in parts of northern England, after a spike in virus cases.
The prime minister said progress against coronavirus continues, with the daily and weekly number of deaths falling, but warned that some European countries are “struggling” to control it. The UK must be ready to “react”, he said.
A further 120 people have died with the virus in the UK, according to the latest figures, and 880 new cases have been recorded.
Highlighting the latest figures from the Office for National Statistics (ONS), Mr Johnson added: “The prevalence of the virus in the community, in England, is likely to be rising for the first time since May.”
He said that the reopening of society had always been “conditional” on “continued progress against the virus”, but with “numbers creeping up” it was time to “squeeze that brake pedal in order to keep the virus under control”.
He urged people to “follow the rules, wash our hands, cover our faces, keep our distance – and get a test if we have symptoms”, summing the advice up with a new slogan: “Hands, face, space, get a test”.
Cases in England are increasing for the first time since May.
Data from the Office for National Statistics shows there are around 4,200 new infections a day compared with 2,800 a week ago.
This is not a return to the height of the epidemic in March, but it is telling.
Every restriction we ease increases the ability of the coronavirus to spread, and the government’s scientific advisers have always warned there was not much wiggle room to lift restrictions and still suppress it.
The uptick in infections is a warning we may have already passed the limit of lifting lockdown.
That is why Boris Johnson has delayed some of the planned lifting of restrictions in England and face masks will become a more frequent sight.
The big question remains around schools. If the current rules are leading to an increase in cases, can we open schools as well? If we open schools will we have to close something else?
It is worth noting all this is happening in July and scientists suspect the virus will spread even more easily in the winter months.
Mr Johnson said the planned reopening of “higher risk settings” on 1 August would now be delayed for at least a fortnight.
This means that the following will not be able to take place until 15 August, at the earliest:
Fans attended the World Snooker Championship when it started on Friday as part of a pilot to test the return of larger crowds to sports venues. The tournament will now go ahead without spectators until at least 15 August, which is when the final is scheduled to begin.
The British Beauty Council said the changes were “very disappointing for a sector that has already seen delay after delay in reopening”.
Separately, face coverings will be compulsory in more indoor settings where people are likely to come into contact with people they do not know, such as museums and places of worship, from next weekend. They are already required in shops, banks, airports and other indoor transport hubs.
The prime minister said the rules for face coverings would become enforceable in law from 8 August.
However, he said the plan to pause shielding for those most vulnerable to the virus will go ahead from Saturday.
That means some 2.2 million people who have been self-isolating in England during the pandemic can return to work, if they cannot work from home, as long as their workplace is Covid-secure.
Guidance for employers will also change as planned from the start of August, providing “more discretion over how employees can work safely, whether by continuing to work from home, or attending a Covid-secure workplace”, Mr Johnson said.
The latest announcement came shortly after new lockdown rules were introduced in parts of England, banning separate households from meeting each other inside their homes and private gardens.
The rules, which came into force at midnight, impact people in Greater Manchester, east Lancashire, parts of West Yorkshire, and in Leicester.
They also ban members of two different households from mixing in pubs and restaurants, although individual households will still be able to visit such hospitality venues.
Health Secretary Matt Hancock told BBC Breakfast that the increase in transmission in the affected regions was due to people visiting friends and relatives, citing fresh data from contact tracing.
The changes come as Muslim communities prepare to celebrate Eid this weekend, and nearly four weeks after restrictions were eased across England – allowing people to meet indoors for the first time since late March.
Speaking about the latest restrictions in parts of northern England, Labour leader Sir Keir Starmer said the government had made the “right decision”, but urged it to “improve” what he called “extremely poor” communication.
Ministers have said police forces and councils will be given powers to enforce the new rules.
In other key developments: