The final stage of easing lockdown restrictions in England is to be delayed until 19 July.
It means most remaining curbs on social contact will continue beyond 21 June, when they had been due to be lifted.
The limit on wedding guest numbers will be removed but venues will still have to adhere to other rules.
Prime Minister Boris Johnson said there would be a review after two weeks and he was “confident” the delay would not need to be longer than four weeks.
However, he told a Downing Street press conference he could not rule out the possibility the date could be pushed back further.
Scientists advising the government had warned of a “significant resurgence” in people needing hospital treatment for Covid-19 if stage four of easing the lockdown went ahead on 21 June.
It comes amid rising cases, driven by the more transmissible Delta variant, which was first identified in India.
Mr Johnson said going ahead with stage four on 21 June would mean “a real possibility” of the virus outrunning the vaccines, leading to thousands more deaths which could otherwise have been avoided.
The delay would give the NHS “a few more crucial weeks” to get people vaccinated, he said, adding that while the link between infections and hospital admissions had been “weakened” it had not been “severed”.
“We will monitor the position every day and if, after two weeks, we have concluded that the risk has diminished, then we reserve the possibility of proceeding to step four and a full opening sooner,” he said.
“At a certain stage, we are going to have to learn to live with the virus and to manage it as best we can,” he added.
Addressing MPs in the Commons later, Health Secretary Matt Hancock said the decision not to ease restrictions next week was being made with a “heavy heart” but the government’s four tests for easing restrictions – one of which is that the risks are not fundamentally changed by new variants – had not been met.
He said extra testing facilities and access to vaccines would be rolled out in more areas of the country, while vaccinations would be opened up on Tuesday in England to people aged 23 and 24.
