The planned easing of lockdown restrictions in England has been postponed for at least a fortnight- with face masks to become mandatory by law in all public indoor settings on 8 August.
Boris Johnson told a Downing Street briefing that the decision to "squeeze that brake pedal" on changes had been taken due to coronavirus infection numbers "creeping up" and "in order to keep the virus under control".
England's Chief Medical Officer Chris Whitty, meanwhile, warned the country had "probably reached near the limits, or the limits, of what we can do in terms of opening up society" without causing a further spike in COVID-19 cases.
It means plans due to come in to force on Saturday to reopen casinos, bowling alleys and skating rinks will not now happen until 15 August at the earliest.
In addition, 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, as had been planned.
The prime minister also announced that rules around face coverings would be extended to make them mandatory in all public indoor settings, "such as museums, galleries, cinemas and places of worship".
He said this measure would become enforceable in law from 8 August.
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"Most people in this country are following the rules and doing their best to control the virus," he said.
"But we must keep our discipline and our focus and we cannot be complacent.
"I have asked the Home Secretary to work with the police and others to ensure the rules which are already in place are properly enforced."
He said this would mean "a greater police presence to ensure face coverings are being worn where this is required".
"At this stage, we are not changing the rules on social contact nationally," he added, before warning a further tightening of restrictions may be necessary.
"I don't want to tell people to spend less time with their friends," he said.
"But unless people follow the rules and behave safely, we may need to go further."
Mr Whitty said: "We all know that what we have to try and do is to get to the absolute edge of what we can do in terms of opening up society and the economy without getting to the point where the virus starts to take off again.