Video report by ITV News Political Correspondent Libby Wiener
A lifting of lockdown restrictions will not go ahead as planned in England, the prime minister has announced, saying he needed to "react to the first signs of trouble".
Boris Johnson said small wedding receptions must be postponed and the reopening of bowling alleys and casinos must be paused, with the prevalence of coronavirus "likely to be rising for the first time since May".
Such venues and events had been due to restart in England from August 1 but must now be paused until August 15 "at the earliest".
The prime minister also announced that face coverings are to be mandatory in indoor settings where people are likely to come into contact with people they do not know, such as museums and places of worship, from August 8.
Pilots sports events that would have had to have fans inside venues this weekend will also have to be abandoned.
But the reopening of Covid-secure work places should continue.
"We know that employers have gone to huge lengths to make workplaces safe so that guidance remains unchanged," the prime minister said.
"It is safe to get into a Covid-secure workplace and people should understand that and that is our guidance.”
Video report by ITV News correspondent Damon Green
The following will not restart until August 15 'at the earliest':
Casinos, bowling alleys, skating rinks and other close contact services must remain closed
Indoor performances will not resume
Pilots of larger crowds in sports venues and conference centres will not take place
Wedding receptions of up to 30 people will not be permitted, but ceremonies can continue to take place, in line with COVID-Secure guidelines
During the Downing Street press conference Mr Johnson repeated the phrase "hands, face, space" which appears to be a new government slogan instructing people to wash their hands, cover their face and keep to a social distance.
He said the he "only real utensil" people have in controlling the spread of Covid-19 is "human behaviour and the only way we can encourage people to behave in one way or the other is to through advice".
"That was pretty punchy I think – ‘hands, face, space, and get a test’," he said, "I think everybody can more or less remember that."
Mr Johnson gave the press conference after it was announced late last on Thursday that meetings between separate households in part of north west England had been banned.
The prime minister has been warning about "signs of a second wave" in Europe and is keen to take "swift and decisive action" to impose restrictions "where we think that the risks are starting to bubble up again".
He warned how one in 1500 people now now have coronavirus, compared to one in 100 on July 15 and one in 2000 on July 2.
Mr Johnson said it was right to respond to a “warning light on the dashboard” by pausing the lifting of some restrictions.
"We cannot be complacent and I won’t stand by and allow this virus to cause more pain and more heartache in our country."
Mr Johnson told a Downing Street briefing: “With those numbers creeping up, our assessment is that we should squeeze that brake pedal… in order to keep the virus under control.
“On Saturday August 1, you’ll remember we had hoped to reopen a number of the higher-risk settings that had remained closed and today I’m saying we’re postponing those changes for at least a fortnight.
“That means until August 15 at the earliest casinos, bowling alleys, skating rinks and the remaining close-contact services must remain closed.
“Indoor performances will not resume, pilots of larger gatherings in sports venues and conference centres will not take place, and wedding receptions of up to 30 people will not be permitted.”
Following the tightening of restrictions in north west England, Scotland First Minister Nicola Sturgeon said she was "strongly advising" against travel to the affected areas.
She also asked "people from these areas not to travel to Scotland".
"If people from Scotland are already there were are not asking you to cut short your stay but we would say that you should comply with all the advice while you're there and when you come home to Scotland be extra vigilant," she added.
Health Secretary Matt Hancock said the UK government was "prepared to take action, sometimes quite quickly" in order to prevent the wave coming to the UK.
But ministers have been attacked by Labour and Muslims over a lack of communication ahead of the last minute announcement.
New rules in Greater Manchester, parts of East Lancashire and parts of West Yorkshire mean people are banned from meeting each other indoors from midnight on Friday.
It has caused huge disruption for thousands of Muslims who had been hoping to celebrate Eid this evening at the homes of their family and friends.
Earlier, Health secretary Matt Hancock defended the swift new measures, saying it was "important to move fast" late on Thursday evening.
"With no action at all, the virus doubles every three days so it is is important to move fast and sometimes that means communicating very rapidly," he told ITV News.
But the Muslim Council of Britain has criticised the government for its lack of communication - which has caused disruption for Muslims around the country who had been hoping to meet with family this weekend.
The group's secretary general Harun Khan, said: "For Muslims in the affected areas, it is like being told they cannot visit family and friends for Christmas on Christmas Eve itself.
“Failure to communicate makes it difficult for communities across the country to continue working together to minimise the spread of the virus, whilst eroding trust in the ability of authorities to steer our course as we tackle the Covid-19 crisis."
Watch Boris Johnson's press conference in full
Mr Johnson said he understood that the latest announcement would be a “real blow” to people who had wedding plans or those who can no longer celebrate Eid in the ways they would wish.
“I’m really, really sorry about that, but we simply cannot take the risk. We will of course study the data carefully and move forward with our intention to open up as soon as we possibly can," he said.
Labour leader Sir Keir Starmer also criticised the government's communication.
Writing on Twitter, he said: “Despite poor communication from the Government, it’s crucial that we all follow the advice to help prevent another outbreak."
The areas affected by the new north west restrictions:
Blackburn with Darwen