The further easing of lockdown restrictions in England - due to come in this weekend - has been postponed for at least two weeks, after an increase in coronavirus cases.
Casinos and bowling alleys will remain shut, as Boris Johnson said it was time to "squeeze the brake pedal".
Face coverings will be mandatory in more indoor settings, such as cinemas.
England's chief medical officer, Prof Chris Whitty, warned the UK may have hit its limits on easing restrictions.
Appearing alongside the prime minister at a special Downing Street briefing, Prof Whitty said 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 fully reopen to all pupils in the autumn, he said it was a "difficult balancing act" but "we have probably reached near the limit, or the limits, of what we can do in terms of opening up society."
Speaking at the briefing, Mr Johnson said planned changes to guidance for those who have been shielding during the pandemic, and advice for employers, will still go ahead.
The rethink on easing England's lockdown follows new restrictions for people in parts of northern England, after a spike in virus cases.
The prime minister said progress in tackling 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.
Figures published by the Office for National Statistics (ONS) on Friday suggest infection rates in England are rising, with around 4,200 new infections a day - compared with 3,200 a week ago.
The ONS's estimates of daily cases - based on a sample of households completing swab tests - are higher than the figures for lab-confirmed cases reported by the Department of Health and Social Care (DHSC) because they include people without symptoms who would not otherwise have applied for a test.
Highlighting the ONS figures, 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 with "numbers creeping up" it was time to "squeeze that brake pedal in order to keep the virus under control".
Folks, if we all remember to do these three things, then we can keep the virus under control.— Boris Johnson (@BorisJohnson) July 31, 2020
Wash your hands, cover your face, and keep your space from those not in your household. #HandsFaceSpace pic.twitter.com/uBuxvvnWKV
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 the slogan: "Hands, face, space, get a test".
A further 120 people have died with Covid-19 in the UK according to the latest DHSC figures, bringing the total number of virus deaths to 46,119. Meanwhile, 880 new lab-confirmed cases have been recorded.
Cases in England are increasing for the first time since May.
Data from the ONS suggests there are around 4,200 new infections a day compared with 3,200 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.
You can read more from James here.
Mr Johnson said the planned reopening of "higher risk settings" on 1 August would 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:
- the reopening of casinos, bowling alleys, skating rinks and remaining close contact services
- indoor performances
- pilots of larger gatherings in sports venues and conference centres
- the expansion of wedding receptions to allow up to 30 people
- beauty treatments that involve the face, such as eyelash, eyebrow and threading treatments
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.
Wedding sector 'itching to get back'
Neil White, 51, from Chorley in Lancashire, owns a wedding photography business. Of the 44 weddings he had scheduled for this year, just three bookings remain."I think I speak for the rest of the wedding industry in that there is a huge amount of worry and stress about the future," he said.
Mr White said that while businesses such as pubs and restaurants in have been able to reopen, those in the wedding sector "seem to have been brushed under the carpet" even though they are "itching to get back to work".
"If it continues to next year there are a lot of businesses that are going to close," he added.
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 be 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, so long as their workplace is Covid-secure.
The latest announcement came shortly after new lockdown rules were introduced in parts of northern England, including Greater Manchester, east Lancashire, and parts of West Yorkshire. The rules include a ban on separate households meeting each other inside their homes and private gardens.
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.
Labour leader Sir Keir Starmer said the government had made the "right decision" regarding the new rules for parts of northern England, 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:
- Pubs and restaurants in Leicester - which saw the UK's first local lockdown - are set to reopen from Monday as a number of restrictions in the city are lifted
- A 14-year-old girl is being treated for a life-threatening illness after being "gravely neglected" by her family during lockdown. The High Court's Family Division has been asked to intervene to take the girl into care
- Thousands of people descended on beaches in Bournemouth, Brighton and Poole, prompting some local councils to warn day-trippers to stay away
- Tory MP Craig Whittaker has defended his claim that some Muslims in West Yorkshire are "not taking the pandemic seriously"
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