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The further easing of coronavirus 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.
And people attending places of worship will also be among those required to wear face coverings, in a change that will be applied from next weekend.
But planned changes to guidance for people who have been shielding during the pandemic, and advice for employers, will still go ahead.
The rethink follows new restrictions for people in parts of northern England, after a spike in virus cases.
At a news conference in Downing Street, 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.
Highlighting the latest figures from the Office for National Statistics, 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 stressed again that the reopening of society was "conditional" on "continued progress against the virus" and that No 10 "wouldn't hesitate to put on the brakes" if required.
The prime minister said that with virus "numbers creeping up", the government had decided to "now 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".
Also at Friday's news conference, England's chief medical officer Prof Chris Whitty warned that it might not be possible to further ease lockdown.
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 "idea that we can open up everything and keep the virus under control" is wrong, he said, but it is clear that "schools are an absolute priority" for society.
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.
You can read more from James here.
Mr Johnson said planned reopenings for 1 August would be delayed for at least a fortnight.
That means venues such as casinos, bowling alleys, skating rinks and remaining close contact services must remain closed until 15 August.
Indoor performances will also not resume, pilots of larger gatherings in sports venues and conference centres will not take place, and the planned expansion of wedding receptions to allow up to 30 people will not be permitted.
Spectators were due to watch the first two days of two county cricket matches, while fans were at the World Snooker Championship when it started on Friday. The tournament will now go ahead without spectators until at least 15 August, which is when the final is scheduled to begin.
Image copyright AFP Image caption Fans will no longer be allowed to attend pilot sports events scheduled for this weekend in EnglandBeauty salons will also be unable to start offering treatments that involve the face, such as eyelash, eyebrow and threading treatments.
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.
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However, the plan to pause shielding for the people most vulnerable to the effects of the virus will go ahead as planned from Saturday, according to the prime minister.
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, giving employers "more discretion over how employees can work safely, whether by continuing to work from home, or attending a Covid-secure workplace", Mr Johnson said.
According to the ONS, there is "now evidence to suggest a slight increase in the number of people in England testing positive on a nose and throat swab in recent weeks".
A sample of households in England, excluding care homes and hospitals, were swabbed to test for current infection.
The ONS said daily cases had risen from an estimated 2,800 to 4,200 since last week.
However there is not enough data to suggest a higher proportion of positive tests in any particular region.
The latest announcement comes within hours of new lockdown rules 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 criticised "extremely poor" communication.
He called on No 10 to "improve" communication as "we are going to see more of these situations over the coming weeks and months".
Ministers have said police forces and councils will be given powers to enforce the new rules.
In other key developments:
A further 120 people have died with the virus in the UK, according to the latest figures, and 880 new cases have been recorded The Scottish government has warned residents not to visit areas of England affected by new local lockdown rules Police and council officers are patrolling the seafront in Bournemouth and Poole as thousands descend on the towns' beaches. Local councils, including Brighton and Hove and Thanet - are warning day-trippers to stay away due to large crowds Scotland is developing its own coronavirus contact-tracing app, which it hopes to have ready for use in the autumn In Wales, up to 30 people can meet outdoors and children under 11 will not have to social distance from Monday. Facilities such as swimming pools, gyms and indoor play areas will be allowed to reopen from 10 August Tory MP Craig Whittaker has defended his claim that some Muslims in West Yorkshire are "not taking the pandemic seriously"Areas included in announcement
Cases per 100,000 people
Are you getting married this weekend? Or were you reopening your business? How will the postponement affect you? Share your experiences by emailing haveyoursay@bbc.co.uk.
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