In a last-minute press conference shortly after midday on Friday, July 31, Mr Johnson said progress was being made in the fight against the virus – but that the UK was ‘not exempt’ from a rise in cases as other countries begin to feel the impact of a potential second wave.
He told reporters: “I’ve also consistently warned that this virus could come back and that we would not hesitate to take swift and decisive action as required.
“I’m afraid that in parts of Asia and in Latin America, the virus is gathering pace and some of our European friends are also struggling to keep it under control.
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“As we see these rises around the world, we can’t fool ourselves that we are exempt. We must be willing to react to the first signs of trouble.”
The planned reopenings due to take place on Saturday, August 1 - which included casinos, bowling alleys, skating rinks and close-contact services – will now not go ahead.
Instead, they will be postponed for at least a fortnight as the Government continues to monitor the situation.
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 also not be allowed.
The Prime Minister continued: “With those numbers creeping up, our assessment is that we should squeeze that brake pedal… in order to keep the virus under control."
Mr Johnson also announced at the conference that there has been an increase in the number of people testing positive for Covid-19 in England.
According to the Office for National Statistics (ONS), between Monday, July 20 and Sunday, July 26, there were around 0.78 new Covid-19 infections for every 10,000 people in the community population in England.
This is equal to around 4,200 new cases per day. This is up from an estimated 2,800 new cases a day in the previous week.
Guidance for employers making workplaces Covid-secure for their staff to return from August 1 and a pause to the shielding programme from the same date remain unchanged.
Face coverings will also become mandatory in indoor settings where people are likely to come into contact with people they do not know, it was announced on Friday.
This includes museums, places of worship and cinemas from Saturday, August 8.
The Prime Minister added: “At this stage, we are not changing the rules on social contact nationally. I don’t want to tell people to spend less time with their friends.
"But unless people follow the rules and behave safely, we may need to go further.
“Two weeks ago, I said we would hope for the best but plan for the worst.”
The press conference followed an announcement from Health Secretary Matt Hancock on Thursday, July 31, which saw a change to lockdown restrictions in some parts of northern England due to a rise in Covid cases there.
This covers Greater Manchester, parts of east Lancashire and West Yorkshire.